We calculated the luminosity at 3,000Å before [Lλ(3,000Å)tot] and after the Balmer continuum [Lλ(3,000Å)pow] contribution was subtracted. In the sample of 287 Type 1 AGN spectra, the Balmer continuum contributes to the continuum Lλ(3,000Å) on average for ∼10%, with the maximal value of 18%. The ratios of the Lλ(3,000Å), with and without the Balmer continuum contribution, are shown on the histogram (**Figure 2A**). The majority of the equations for the MBH estimation using the UV parameters neglect the contribution of the Balmer continuum (see e.g., McLure and Jarvis, 2002; Vestergaard and Osmer, 2009). Wang et al. (2009) give the relation for the assessment of MBH using the FWHM Mg II and Lλ(3,000Å), assuming that after subtraction of the Balmer continuum, the Lλ(3,000Å) is the pure power law continuum, therefore we then compared MBH calculated with and without consideration of the Balmer continuum contribution. We got that the Balmer continuum increased MBH on average for ∼5% (0.02 dex), with the maximal value of the MBH overestimation up to 10% (0.04 dex). The ratio of the MBH estimates before and after the Balmer continuum subtraction is shown in **Figure 2B**. The comparison of MBH values before (index "tot") and after the Balmer continuum subtractions (index "pow") are also given in **Figure 3**, where the correlation coefficients ρ (ρtot = 0.783 and ρpow = 0.786), P-values (Ptot = 0 and Ppow = 0) and linear best-fit (y = a ∗ x+b) coefficients are given: slope a, (atot = 0.85 and apow = 0.84) and y-intercept b, (btot = 1.36 and bpow = 1.43). It can be seen that the influence of the Balmer continuum to the MBH estimation is so small that it barely changes the correlation coefficient (ρ) or the coefficients of the linear bestfit (a, b) between the MBH estimated with the optical and the one estimated with the UV parameters. Also, it seams that removing the Balmer continuum does not affect the outliers in this relationship.

# 4. CONCLUSIONS

Here we have used the sample of the SDSS Type 1 AGN spectra to compare the most frequently used emission lines for the MBH estimation, Hβ (in the optical band) and Mg II (in the UV band), in order to assess which line is better virial estimator and thus more convenient for that purpose. We investigated how the Balmer continuum affect the BH mass estimation using the UV parameters.

From our investigation we can outline the following conclusions:

# REFERENCES

At the end, let us note that similar investigation should be performed on the sample where more reliable methods for mass measurements (as e.g., reverberation) should be applied to explore the influence of the Balmer continuum and this we postpone for our future work. Moreover, some additional effects (as e.g., relativistic jets) can significantly affect line profiles, i.e., the radio loudness which can indicate the presence of relativistic jets, therefore in the future work radio properties of the sample should also be explored.

# AUTHOR CONTRIBUTIONS

JK, SM, and LP: Developing concept of the work; the acquisition, analysis and interpretation of data for the work; drafting the work and revising it critically for important intellectual content; final approval of the version to be published; being accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

## ACKNOWLEDGMENTS

The work is a part of the project 176001 financed by the Ministry of Education, Science, Technology and Development, Republic of Serbia.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2017 Kovaˇcevi´c-Dojˇcinovi´c, Marˇceta-Mandi´c and Popovi´c. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Ultraviolet/Optical Emission of the Ionized Gas in AGN: Diagnostics of the Ionizing Source and Gas Properties

Anna Feltre1, 2 \*, Stephane Charlot 2 , Marco Mignoli 3 , Angela Bongiorno4 , Francesco Calura3 , Jacopo Chevallard5 , Emma Curtis-Lake2 , Roberto Gilli 3 and Adele Plat 2

1 Univ Lyon, Univ Lyon1, Ens de Lyon, Centre National de la Recherche Scientifique, Centre de Recherche Astrophysique de Lyon UMR5574, Saint-Genis-Laval, France, 2 Sorbonne Universités, UPMC-Centre National de la Recherche Scientifique, UMR7095, Institut d'Astrophysique de Paris, Paris, France, 3 INAF-Osservatorio Astronomico di Bologna, Bologna, Italy, 4 INAF-Osservatorio Astronomico di Roma, Monteporzio Catone, Italy, 5 Scientific Support Office, Directorate of Science and Robotic Exploration, European Space Research and Technology Centre (ESTEC), European Space Agency (ESA), Noordwijk, Netherlands

#### Edited by:

Paola Marziani, Osservatorio Astronomico di Padova (INAF), Italy

#### Reviewed by:

José María Solanes, University of Barcelona, Spain Milan S. Dimitrijevic, Astronomical Observatory, Serbia

> \*Correspondence: Anna Feltre<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Milky Way and Galaxies, a section of the journal Frontiers in Astronomy and Space Sciences

Received: 30 August 2017 Accepted: 19 October 2017 Published: 02 November 2017

#### Citation:

Feltre A, Charlot S, Mignoli M, Bongiorno A, Calura F, Chevallard J, Curtis-Lake E, Gilli R and Plat A (2017) Ultraviolet/Optical Emission of the Ionized Gas in AGN: Diagnostics of the Ionizing Source and Gas Properties. Front. Astron. Space Sci. 4:32. doi: 10.3389/fspas.2017.00032 Spectroscopic studies of active galactic nuclei (AGN) are powerful means of probing the physical properties of the ionized gas within them. In particular, near future observational facilities, such as the James Webb Space Telescope (JWST), will allow detailed statistical studies of rest-frame ultraviolet and optical spectral features of the very distant AGN with unprecedented accuracy. In this proceedings, we discuss the various ways of exploiting new dedicated photoionization models of the narrow-line emitting regions (NLR) of AGN for the interpretation of forthcoming revolutionary datasets.

Keywords: active galaxies, emission lines, ultraviolet, spectral models, spectroscopy

# 1. INTRODUCTION

Nebular emission lines observed in galaxy spectra contain valuable information about the nature of the ionizing source and physical conditions of the ionized gas within these same galaxies. Current (e.g., VLT-KMOS/MUSE, Keck-MOSFIRE) and future (e.g., the Near Infrared Spectrograph, NIRSpec, on-board JWST) near infrared spectrographs will provide ultraviolet, in addition to optical, rest-frame spectra of galaxies out to the epoch of Reionization. In this context, it is extremely important to develop physically motivated spectral models, along with analysis tools based on advanced statistical techniques, for the interpretation of the rest-frame optical/ultraviolet spectra of both active and inactive galaxies at all cosmic epochs. In section 2 we describe photoionization models of the AGN NLR. In the following Sections we show how (i) new ultraviolet, in addition to standard optical, spectral diagnostic diagrams allow one to distinguish between nuclear activity and star formation, (ii) these new models can be best used to understand the physical properties of the gas in the AGN NLR and (iii) the implementation of these AGN photoionization calculations in an innovative bayesian fitting code can help us best interpret current, and future, spectro-photometric data on active galaxies.

# 2. SPECTRAL MODELS

The nebular emission of the AGN NLR is computed combining the AGN ionizing spectrum, described as a series of broken power laws (Equation 5 of Feltre et al., 2016), with the photoionization code CLOUDY (version c13.03, latest described in Ferland et al., 2013). The gas in the NLR is modeled with clouds of single type using the approach described in Feltre et al. (2016) 1 . The models are parametrized in terms of (i) the ultraviolet spectral index, α, of the incident radiation field, (ii) the ionization parameter, (iii) the hydrogen gas density of the clouds, (iv) the gas metallicity, and (v) the dust-to-heavy element mass ratio. A more detailed explanation of the physical parameters is provided in Feltre et al. (2016). Note that we have improved the original model grid by adding two new adjustable parameters, namely the inner radius of the NLR and the internal microturbulence velocity of the gas cloud. The addition of these two parameters have been found to be critical for reproducing high ionization emission-lines, such as N V λ1240 and C IV λ1550 (Mignoli et al., in prep., Feltre et al., in prep.). We also consider new generation photoionization calculations of the nebular emission from stars (Gutkin et al., 2016), parametrized in analogous way to the AGN NLR models. We remind the reader to the papers of Charlot and Longhetti (2001), Gutkin et al. (2016) for a detailed description of these spectral models.

# 3. DIAGNOSTIC DIAGRAMS IN THE ULTRAVIOLET

Standard diagnostic diagrams, based on ratios of optical emission-lines (such as [O III]λ5007, [N II]λ6584, [S II]λ6724, Hα and Hβ) are commonly used to distinguish between stellar and nuclear activity (e.g., Baldwin et al., 1981; Veilleux and Osterbrock, 1987). In addition to optical, we explored new diagnostics at ultraviolet wavelengths for three reasons: (i) future facilities will provide high quality rest-ultraviolet spectra of the most distant sources, (ii) models are usually calibrated on optical observations of the local Universe and this might not always be appropriated to study the emission from star-formation and interstellar gas at high redshift, and (iii) standard optical diagnostic diagrams might fail to distinguish between stellar and AGN activity at higher redshift (e.g., in the case of low metallicity Groves et al., 2006; Coil et al., 2015; Feltre et al., 2016; Hirschmann et al., 2017). Diagrams involving combinations of a collisionally excited metal line or line multiplet, such as C IV λλ1548, 1551, O III]λλ1661, 1666, N III]λ1750, [Si III]λ1883+Si III]λ1892 and [C III]λ1907+C III]λ1909, with the He II λ1640 recombination line have been found to allow a good distinction of the nature of the ionizing source as well as valuable constraints on interstellar gas parameters and the shape of the ionizing radiation (Feltre et al., 2016). **Figure 1** shows the predictions from photoionization models of both AGN and star-forming galaxies, for different values of metallicity and ionization parameter in two diagnostic diagrams, namely C III]λ1909/He II λ1640 vs. C IV λ1550/He II λ1640 (left panel) and C IV λ1550/C III]λ1909 vs. C IV λ1550/He II λ1640 (right panel). Note that model predictions agree well with data measurements of AGN (Dors et al., 2014) and star-forming galaxies (Stark et al., 2014) currently available in the literature.

# 4. GAS METALLICITY IN THE NLR OF Z∼2 TYPE 2 AGN

As case study, to illustrate a potential application of photoionization models described above (section 2), we compare the model predictions with the emission line measurements of a sample of C IV λ1550-selected Type 2 AGN (see section 4.1). The ionized gas in the NLR of AGN surrounds the black hole at the galaxy centre and it is likely connected to the nuclear star formation. By measuring the metal abundance of the ionized gas in the AGN NLR, we can obtain indirect information about the star formation history of the host galaxy.

# 4.1. Sample Description

To pursue our goals we appeal to VIMOS observations from the z-COSMOS Deep Survey (Lilly et al., 2007). We limited our study to the redshift range 1.45 < z < 3.05 to assure that the C IV λ1550 emission line is well covered by the observed spectral range. The presence of this feature in a galaxy spectrum is indicative of nuclear activity. We identified 192 C IV λ1550 selected objects, i.e., sources with a C IV λ1550 line intensity 5 times larger than the significance level. Out of these 192, we classified 90 sources as Type 2, i.e., narrow line, AGN (i.e., full width half maximum, FWHM, of the C IV λ1550 line lower than 2,000 km/s). From the rest-frame ultraviolet spectra of our sample of Type 2 AGN we measured fluxes, velocity dispersions and equivalent widths of the emission lines. Note that the AGNselection effectiveness has been also confirmed by the ultraviolet diagnostic diagrams. The spectral observations, sample selection and data measurements will be presented in further details in Mignoli et al., in prep.

# 4.2. Gas Metallicity from Ultraviolet Emission-Lines

The C IV λ1550selected Type 2 AGN sample, described in section 4.1, is ideal to estimate the metal content of the ionized gas in the AGN NLR thanks to the simultaneous presence of two or more emission lines of Oxygen, Nitrogen and/or Carbon ions in the same spectra, along with the plethora of the other ultraviolet emission lines. In particular, we consider ratios of any possibile combination of the N V λ1240, N IV]λ1485, C IV λ1550, He II λ1640, C III]λ1909, C II]λ2326 and [Ne IV]λ2424 emission lines. We perform a spectral fitting by applying a simple least square minimization2 to infer the total Oxygen (both gas and dust phase) abundance, expressed as 12 + log(O/H)3 . Note that, further improvements will include the exploitation of a new generation fitting tool, BEAGLE (Chevallard and Charlot, 2016, Chevallard et al., in prep.), based on sophisticated bayesian statistic techniques (see section 5).

1Predictions of the intensities of the main optical and ultraviolet emission-lines are available through http://www.iap.fr/neogal/models.html

2Note that we consider a galactic attenuation curve (Cardelli et al., 1989), for consistency with the fitting procedure used to derive the host galaxy stellar masses. 3For a direct translation between metallicity, Z, and Oxygen abundance we remind to Table 2 of Gutkin et al. (2016), where solar metallicity Z⊙ = 0.01524 correspond to 12 + log(O/H) = 8.83

We also compute the stellar masses of the host galaxy by appealing to a multi-band spectral energy distribution (SED) fitting technique (Bongiorno et al., 2012). A combination of AGN and host-galaxy templates is used to fit the large set of optical and near-infrared photometry from the Cosmic Evolution Survey (COSMOS; Scoville et al., 2007) available for our sources. The left panel of **Figure 2** shows the NLR gas metallicity, 12+log(O/H), vs. the stellar mass. The first thing to note is that the fit favors models with solar (black dashed lines) or slightly subsolar metal content. This is in contrast with previous findings, where the N V λ1240/He II λ1640 ratio has been often found to be stronger than model predictions. Among the solutions, proposed in the literature, to explain this high N V λ1240/He II λ1640 ratio, there are very high metallicities (up to 5–10 time solar metallicity) and "selectively" enhanced Nitrogen (e.g., Hamann and Ferland, 1992, 1993; Shemmer and Netzer, 2002; Nagao et al., 2006). Solving this problem was one of the main reason for the update of the AGN NRL models of Feltre et al. (2016) with additional physical parameters (i.e., NLR inner radius and internally microturbulent clouds). We also do not observed any correlation between the metallicity and the stellar mass of the host galaxy. One can interpret this as a missing link between the gas in the NLR and the interstellar gas in the galaxy. Nevertheless, our analysis is based on a restricted range of stellar masses and the stellar mass host estimates depends both on the templates and fitting technique used for the analysis. To have a more comprehensive analysis, we plan to derive both metallicity and stellar masses from the same fitting technique, based on advanced statistical methods (see section 5).

The right panel of **Figure 2** suggests a decrease of metallicity with increasing redshift. Despite previous findings in the literature, where no evidence for a redshift evolution of the metallicity in AGN was found (Nagao et al., 2006), a lower metal content moving in more distant sources is what one would expect from models of cosmic chemical evolution. Note also, that the observed trend in **Figure 2** is opposite to the trend, commonly observed in the literature, introduced by the presence of the N V λ1240 line which was favoring very high metallicities, as discussed above.

# 5. SED FITTING TOOL

Future plans include the implementation of the AGN NLR models in a fitting tool based on sophisticated fitting technique, i.e., BEAGLE (Chevallard and Charlot, 2016, Chevallard et al., in prep.). Briefly, BEAGLE combines in a coherent way emission from different components (stars, gas, dust, AGN), adopts Bayesian approach to obtain posterior probability distribution functions of every model parameter and includes predictions from galaxy formation models. BEAGLE allows to chose between different options based on the user's aims: (i) to fit spectro-photometric data at ultaviolet to infrared wavelengths, (ii) to create synthetic catalogs of galaxy SEDs and (iii) to the study retrievability of galaxy physical parameters for different type of observations. BEAGLE can handle data from broad-band photometry and/or spectroscopic information (e.g., full spectra, emission line intensities or equivalent widths). With the implementation of the AGN NRL photoionization models, BEAGLE will be an ideal tool to (i) fit ultraviolet/optical spectra of obscured AGN at any redshift, (ii) study the effects of the presence of AGN with

different accretion luminosities on the ultraviolet/optical spectral features of a galaxy spectrum and (iii) to produce mock catalogs of ultraviolet/optical spectra of Type 2 AGN.

## 6. CONCLUSIONS

In the previous Sections we have described new photoinization calculations for the NLR of AGN. We showed how ratios of emission-lines at ultraviolet wavelengths are good diagnostics of the ionizing source (nuclear vs. stellar activity). Moreover, photoionization models are useful tools for the study of the physical properties of the ionized gas (e.g., metallicity, density) of AGN and they can be easily implemented in fitting tools to interpret current spectroscopic observations of high redshift sources. The exploitation of these spectral models and analysis tools will be particularly useful to interpret observations of highredshift galaxies with future facilities, such as the James Webb Space Telescope and extremely large ground-based telescopes, which will push previous studies up to the epoch of reionization (z>7).

## REFERENCES

# AUTHOR CONTRIBUTIONS

AF and SC developed the spectral models and the diagnostic diagrams. MM assembled the sample, performed the spectral measurements and coordinated the analysis, in collaboration with AF, AB, FC, and RG. AF, AP, EC, JC, and SC all contributed at the implementation of the AGN module within the fitting code BEAGLE.

## ACKNOWLEDGMENTS

AF, AP, EC, and SC acknowledge support from the ERC via an Advanced Grant under grant agreement no. 321323-NEOGAL. AF acknowledges support from the ERC via an Advanced Grant under grant agreement no. 339659-MUSICOS. Part of this study is based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under programme ID 175.A-0839 (zCOSMOS redshits survey). Part of the analysis presented in this work was done with TOPCAT (http://www.star.bris.ac.uk/~mbt/ topcat/), developed by M. Taylor.

**Conflict of Interest Statement:** The handling Editor declared a shared affiliation, though no other collaboration, with one of the authors, AB.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2017 Feltre, Charlot, Mignoli, Bongiorno, Calura, Chevallard, Curtis-Lake, Gilli and Plat. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# The Relationship between Mg II Broad Emission and Quasar Inclination Angle

Conor Wildy\* and Bozena Czerny

*Center for Theoretical Physics, Polish Academy of Sciences, Warsaw, Poland*

Several observed spectral properties of quasars are believed to be influenced by quasar orientation. In this investigation we examine the effect of orientation on the Mg II line located at 2,798 Å in a sample of 36 radio-loud quasars, with orientation angles having been obtained in a previous study using radio observations. We find no significant relationship between orientation angle and either Mg II line full-width at half-maximum or equivalent width. The lack of correlation with inclination angle contradicts previous studies which also use radio data as a proxy for inclination angle and suggests the Mg II emission region does not occupy a disk-like geometry. The lack of correlation with Mg II equivalent width, however, is reported in at least one previous study. Although the significance is not very strong (86%), there is a possible negative relationship between inclination angle and Fe II strength which, if true, could explain the Fe II anti-correlation with [O III] strength associated with Eigenvector 1. Interestingly, there are objects having almost edge-on inclinations while still exhibiting broad lines. This could be explained by a torus which is either clumpy (allowing sight lines to the central engine) or mis-aligned with the accretion disk.

#### Edited by:

*Mauro D'Onofrio, Universita degli Studi di Padova, Italy*

#### Reviewed by:

*Giovanna Maria Stirpe, Osservatorio astronomico di Bologna (INAF), Italy Milan S. Dimitrijevic, Belgrade Astronomical Observatory, Serbia*

\*Correspondence:

*Conor Wildy<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Milky Way and Galaxies, a section of the journal Frontiers in Astronomy and Space Sciences*

Received: *31 August 2017* Accepted: *30 October 2017* Published: *14 November 2017*

#### Citation:

*Wildy C and Czerny B (2017) The Relationship between Mg II Broad Emission and Quasar Inclination Angle Front. Astron. Space Sci. 4:43. doi: 10.3389/fspas.2017.00043* Keywords: active galaxies, accretion, broad line region, Mg II, spectroscopy

# 1. INTRODUCTION

Active Galactic Nuclei (AGN) are luminous sources powered by accretion onto supermassive black holes at the centres of galaxies. Orientation angles are known to affect the observed properties of AGN. For example it is thought that Type 1 objects, which show broad lines in their unpolarized spectra, have a low inclination angle with respect to the accretion disk axis and therefore enable an unobscured view of the central engine, while Type 2 objects, which only exhibit narrow lines, are thought to be viewed at high inclination and therefore obscured by a dusty torus oriented such that it is co-planar with the accretion disk (Urry and Padovani, 1995). This is supported by the detection of broad lines in polarized light from Type 2 objects (Miller and Goodrich, 1990) and higher column density x-ray absorbers in Type 2 objects compared with Type 1 (Turner and Pounds, 1989). It is plausible that the inclination angle can influence the broad line morphology within the group of Type 1 objects. Studies have already indicated this for Hβ in multi-quasar samples, where the width of the broad emission line is dependent on the inclination angle such that at higher inclinations the line tends to be broader (Wills and Browne, 1986; Runnoe et al., 2013). Such a phenomenon is easily explained by assuming that the bulk of the broad line region (BLR) is located in a flattened disk-like structure, with the observed velocity of an individual BLR cloud (vobs) being related to its intrinsic velocity (vint) by vobs = vintsinθi , where θi is the inclination angle.

Quasars are a luminous sub-category of AGN mostly exhibiting Type 1 features. Many thousands have their observed optical spectra catalogued within the Sloan Digital Sky Survey (SDSS) (Adelman-McCarthy et al., 2008), enabling statistical analysis of large samples of these objects. One such example relevant to quasar inclination angles was that of Shen and Ho (2014) which examined the so-called Eigenvector 1 correlations originally described by Boroson and Green (1992). This correlation is mainly due to the inverse relationship between the strengths of Fe II and the optical narrow lines of [O III], and also shows an inverse relationship between the width of Hβ and its strength. The paper of Shen and Ho (2014) showed that, in a sample of approximately 20,000 objects, [O III] strength is roughly constant at a given Fe II strength, while the full-width at half-maximum (FWHM) of Hβ varies significantly between objects. That paper concluded that this variation in FWHM was due to orientation, while the strength anti-correlation of Fe II with [O III] was not. It was described in Risaliti et al. (2011) that, since the emission of [O III] is probably isotropic and scales with the quasar bolometric luminosity, then it is plausible that [O III] EW tracks the quasar inclination angle, with higher EW indicating higher inclination angle. This was further supported in that paper by examination of the shape of the distribution of EW values. A recent study by Bisogni et al. (2017), revealed that the Mg II equivalent width (EW) was not dependent on [O III] EW, suggesting that the emission of Mg II, unlike the case of [O III],

#### TABLE 1 | List of quasars used for Mg II analysis.

was anisotropic. This is consistent with the postulate that BLR emission is located in a disk-like structure.

In this investigation we test the dependence of Mg II width and strength on inclination angle. To do so we selected objects from those listed in Kuzmicz and Jamrozy (2012) ´ , which have direct measurements of quasar orientation obtained from radio data and whose spectra are archived in the SDSS. We improve on the multi-component fit of the underlying continuum in that paper by including the Balmer continuum emission as well as the power-law continuum and broadened Fe II templates, allowing us to obtain more accurate Mg II EW measurements and profile shapes.

#### 2. OBJECTS USED IN THE QUASAR SAMPLE

There are 91 quasars in the Kuzmicz and Jamrozy (2012) ´ paper with optical and radio observations obtained from previous catalogs (see references therein), for which inclination angle measurements were obtainable. Inclination angles were calculated in that study using the follow equation:

$$\theta\_{\dot{\imath}} = \arccos\left(\frac{1}{\beta\_{\dot{\jmath}}} \frac{(\mathbf{s} - \mathbf{l})}{(\mathbf{s} + \mathbf{l})}\right),\tag{1}$$

where s = (Sj/Scj) 1 (2−α) , Sj and Scj are the peak flux density of the lobes appearing closer to and further from the core respectively and α is the spectral index which is assumed to be α = −0.6 from Wardle and Aaron (1997). The constant βj is the jet velocity which, in agreement with Wardle and Aaron (1997) and Arshakian and Longair (2004), was fixed at 0.6c.

Of the 91 quasars are included 43 objects which are categorized as Giant Radio Quasars (GRQs), defined as objects having a radio structure >0.72 Mpc after applying the cosmological parameters of Spergel et al. (2003). As that study was designed to reveal any differences between GRQs and the rest of the radio-loud quasar population, they also selected a comparison sample made up of 48 objects having a smaller radio structure. As their study found on average no statistically significant differences in quasar optical spectra between the two populations, we made no distinction between the two categories and treated all objects as one sample. Only objects having SDSS spectral coverage spanning the entire range 2,300–3,300 Å in the quasar rest frame were subject to analysis, as this allows an adequate span to fit the underlying emission made up of the power-law continuum from the accretion disk, broadened Fe II emission and Balmer continuum emission from the BLR. Finally, only 36 objects from the sample underwent scientific analysis due to: (i) five objects having no SDSS spectra; (ii) 30 objects not meeting the 2,300–3,300 Å criterion; (iii) one object having heavily absorbed Mg II emission; and (iv) 19 objects being unable to be fitted satisfactorily. These 36 objects and their spectral parameters measured in this study are listed in **Table 1**. All EW values are calculated with respect to the disk power-law continuum.

# 3. EMISSION COMPONENTS IN THE SPECTRAL REGION OF MG II

The Mg II line exists in a complex spectral region of the UV known as the "small blue bump" (SBB), whose underlying emission must be accurately reconstructed in order to correctly measure the Mg II strength and width. The SBB consists of a BLR Balmer continuum and Fe II emission made up of many overlapping broadened lines which, as they are blended, give the appearance of continuous emission. The SBB adds to the underlying power-law continuum to give the total continuous emission spanning the wavelengths occupied by the Mg II line. This line itself is modelled using 1 or 2 Gaussian profiles, in the two profile case no significance is assigned to the properties of the individual components, they are used simply to generate the total profile from their sum. The modelled Fe II emission is based on the template of Tsuzuki et al. (2006). The calculation of the Balmer continuum was not performed in the fitting procedure of Kuzmicz and Jamrozy (2012) ´ , hence our fitting method represents an important improvement over their technique. The Balmer continuum was calculated using the following equation obtained from Grandi (1982):

$$F\_{\nu}^{\rm BC} = F\_{\nu}^{\rm BE} e^{\left(-h - \nu\_{\rm BE}\right)/\left(kT\_c\right)}\,,\tag{2}$$

where F BC ν is the BC flux at frequency ν, F BE ν is the flux of the BC at the Balmer edge (3,646 Å), Te is the electron temperature, k is the Boltzmann constant and h is the Planck constant. The fitting of all three SBB components was performed in a four-step manner for each quasar. First, the power-law continuum was fitted to relatively line-free regions outside the approximate extent of the SBB (less than 2,300 Å and greater than 3,700 Å) using the specfit software within the Image Reduction and Analysis Facility (IRAF). Second, the Fe II template and Balmer continuum were fitted to spectral regions spanning 2,300–2,600 Å and 3,000–3,300 Å using chi-square minimization, after subtraction of the power-law continuum. These wavelength ranges were used as they are neighbouring bands on either side of the Mg II line. The electron temperature of the plasma generating the Balmer continuum was treated as a free parameter and allowed to vary between 4,000 and 20,000 K in steps of 1,000 K, as was the Balmer edge flux which varied in steps of 0.01% of the continuum subtracted quasar flux. For the Fe II template, the free parameters were Gaussian broadening of between 0 and 3,000 km s−1 in steps of 1,000 km s−1 and the normalization, which varied in steps of 0.01% of the continuum subtracted quasar flux. Third, the Mg II model was fitted to each quasar spectrum with the power-law and SBB continuum subtracted, using specfit. Fourth, all four components were simultaneously re-fitted to the region 2,300–3,300 Å using specfit, with only normalizations allowed to vary. An image of an example resultant fit is shown in **Figure 1**.

## 4. ANALYSIS

## 4.1. Relationship between Mg II Width and Quasar Inclination Angle

A Pearson test for correlation between the Mg II FWHM and the quasar inclination angle for the 36 objects was

performed. This found a probability of 38% that there is an actual correlation between the two variables, indicating that the data is consistent with the null hypothesis (no correlation). Each quasar's FWHM and inclination angle, along with the best-fitting linear trend-line, is illustrated in **Figure 2**.

# 4.2. Relationship between Equivalent Width and Inclination Angle

Similar to the procedure for Mg II FWHM, the EW of the line was tested across all 36 quasars using a Pearson test for correlation. This provided a value of 38% likelihood of correlation. Again, this is consistent with the null hypothesis of no correlation. The

Fe II EW was also measured over the same wavelength span as the Mg II line, revealing that there may be a hint of a correlation between the Fe II EW and inclination angle since the Pearson test indicates an 86% likelihood of correlation, much higher than for the other cases mentioned so far. If such a correlation does in fact exist, it is likely to be a negative relationship given that the correlation coefficient is negative. The relationship of both Mg II and Fe II EW as a function of inclination angle is illustrated in **Figure 3**.

# 4.3. Broad Line Detection at Large Inclination Angles

One of the most interesting findings from the sample is the detection of broad emission lines at almost edge-on inclinations. This is somewhat surprising as it could be expected that at high inclinations the dusty torus obscures the view to the central source of the quasar. There are several examples of such objects in our sample, one of which is shown in **Figure 4**.

# 5. DISCUSSION

The fact that we do not detect a correlation between the Mg II broad line width in our sample and the inclination angle is a somewhat surprising result which contradicts reports in other studies. For example, Aars et al. (2005) found a correlation between the radio-derived inclination angle and the Mg II FWHM, which is in accordance with what would be expected if the broad line region, including the Mg II emitting gas, were located in a flattened-disk geometry. A flattened BLR is further supported by the study of Kimball et al. (2011) which found no correlation between the Mg II EW and the ratio of radio core flux to lobe flux, the latter being an inclination indicator. If instead a dependence between these two values was to have been found, this would imply that the broad line emission is substantially more isotropic than that of the accretion disk and therefore that the BLR cannot have a disk geometry. In the case of [O III] the line emission is isotropic and therefore its EW could potentially track the inclination angle (Risaliti et al., 2011).

Like Kimball et al. (2011) our findings also indicate a lack of correlation between Mg II EW and inclination angle. Considering the lack of correlation of either line width or EW with inclination in our study, it is possible that the BLR exists in an intermediate geometry between an isotropically emitting sphere and an anisotropically emitting thin-disk. Possibilities for such geometries include a thick disk, a distorted disk, or some combination of both. This could result in only a weak relationship between inclination and line width, rendering it undetectable in our sample size, while still presenting no strong evidence of isotropic emission from the relationship between inclination angle and line EW. This could explain the correlation between [O III] EW and broad line width seen in Risaliti et al. (2011), as their sample size is much larger than ours, consisting of approximately 6,000 SDSS quasars.

There is no strong indication of a correlation between Fe II EW and inclination (such a conclusion would require a confidence >95%), however it is stronger than the others, at 86%. This correlation, if it exists, is negative. If the [O III] emission is an accurate indicator of inclination, then this would support the findings of Bisogni et al. (2017) and may be a contributor to the anti-correlation between [O III] and Fe II strength found from the Eigenvector 1 relationship (Boroson and Green, 1992). A possible physical explanation for this, if the anti-correlation is

true, is the Fe II emission being located in a disk which is even thinner than the accretion disk (Bisogni et al., 2017), however this is speculative. It is not possible to accurately measure the Fe II line widths using our method as there are only three values of broadening used and in many cases more than one value gives an acceptable fit to the spectrum.

The fact that broad lines are visible at high inclinations requires an explanation, since, in the common unification explanation for different AGN types, Type 1 Seyferts/quasars should have an inclination angle lower than approximately 45◦ . Broad lines are only visible in Type 2 objects in polarized light (Antonucci and Miller, 1985). In fact, most of the objects studied here have inclination angles greater than 45◦ . If the torus is not smooth but is instead a clumpy structure, then this may allow a line-of-sight toward the central engine in the objects observed. In such a case the probability of detecting a broad line doesn't reach zero even if the inclination angle tends toward 90◦ . Gas inflow models have given support to a clumpy nature of the dusty torus (Netzer, 2015), providing evidence that this structure is possible. A further possibility is that of the torus being misaligned with the accretion disk in these cases. It should be noted in any case that the Kuzmicz and Jamrozy (2012) ´ selection process necessarily found quasars at high inclination angle, since the selection criteria required objects to have both a large angular size and to be lobe-dominated. Quasars of this kind are very rare in

#### REFERENCES

the general quasar population (de Vries et al., 2006) and so our sample objects may have unusual properties compared to those in previous quasar samples compiled to determine the effects of inclination angle on spectral morphology.

# AUTHOR CONTRIBUTIONS

CW was responsible for obtaining and analyzing the data from the SDSS, as well as interpreting the results. BC devised and supervised the project.

#### FUNDING

This project was supported by the Polish Funding Agency National Science Centre, project 2015/17/B/ST9/03436/ (OPUS 9).

#### ACKNOWLEDGMENTS

This work is based on spectroscopic observations made by the Sloan Digital Sky Survey. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2017 Wildy and Czerny. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# On the Intermediate Line Region in AGNs

Tek P. Adhikari 1 \*, Agata Róza˙ nska ´ 1 , Krzysztof Hryniewicz 1 , Bozena Czerny 1, 2 and Gary J. Ferland3

1 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warsaw, Poland, 2 Center for Theoretical Physics, Polish Academy of Sciences, Warsaw, Poland, 3 Department of Physics and Astronomy, The University of Kentucky, Lexington, KY, United States

In this paper we explore the intermediate line region (ILR) by using the photoionisation simulations of the gas clouds present at different radial distances from the center, corresponding to the locations from BLR out to NLR in four types of AGNs. We let for the presence of dust whenever conditions allow for dust existence. All spectral shapes are taken from the recent multi-wavelength campaigns. The cloud density decreases with distance as a power law. We found that the slope of the power law density profile does not affect the line emissivity radial profiles of major emission lines: Hβ, He II, Mg II, C III, and O III. When the density of the cloud at the sublimation radius is as high as 1011.5 cm−3 , the ILR should clearly be seen in the observations independently of the shape of the illuminating radiation. Moreover, our result is valid for low ionization nuclear emission regions of active galaxies.

#### Edited by:

Paola Marziani, Osservatorio Astronomico di Padova (INAF), Italy

#### Reviewed by:

Marco Berton, Università degli Studi di Padova, Italy Milan S. Dimitrijevic, Astronomical Observatory, Serbia

> \*Correspondence: Tek P. Adhikari<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Milky Way and Galaxies, a section of the journal Frontiers in Astronomy and Space Sciences

Received: 25 August 2017 Accepted: 21 September 2017 Published: 29 September 2017

#### Citation:

Adhikari TP, Róza˙ nska A, ´ Hryniewicz K, Czerny B and Ferland GJ (2017) On the Intermediate Line Region in AGNs. Front. Astron. Space Sci. 4:19. doi: 10.3389/fspas.2017.00019 Keywords: active galaxies, emission lines, photoionisation, radiative transfer, accrection disk

# 1. INTRODUCTION

The emission lines in active galactic nuclei (AGNs) provide a unique opportunity to study the properties of the materials located in the environment of the supermassive black hole (SMBH). From the measurement of full width at half maxima (FWHM) of lines in the observed AGN spectra, it is well understood that there exist two separate regions of the line emission. Lines with FWHM ≥ 3,000 km s−1 are emitted by materials with densities ∼1010 cm−3 in the broad line region (BLR) located closer to the AGN central engine. Whereas, the narrow line region (NLR) with gas densities ∼105 cm−3 , located much farther from the AGN center, emits the narrow lines with FWHM ∼500 km s−1 . A clear spatial separation in between BLR and NLR is present where the emission of lines with intermediate FWHM is not seen in the observations.

Theoretically, the lack of emission from the intermediate line region (ILR) was explained by Netzer and Laor (1993, hereafter NL93) as an effect of dust extinction, both in absorption and scattering of line photons and continuum. The authors considered radially distributed clouds of different density and ionization level. Broad and narrow line regions were separated due to the dust content which cannot be present in BLR since the gas temperatures are so high that the dust grains cannot survive there. Nevertheless, further out from the nucleus there is a boundary radius named sublimation radius, Rd, where temperature drops substantially, and dust can sustain up to the distances where NLR is located. NL93 presented that, for assumed gas parameters, the strong drop of line emission appears at distances where potential ILR is expected. Therefore natural separation between BLR and NLR occurs when the dust is taken into account in photoionisation calculations of cloud's emission. This natural separation disclaims the existence of the ILR. In the ILR, the ionization parameter is higher than in further located NLR clouds and hence the relative effect of the dust absorption is stronger. The dust suppressed emission in ILR reappears on transition to the NLR when dust absortion becomes negligible due to the low gas temperature. However, in the recent observations of some AGNs, additional intermediate line component of FWHM ∼700–1,200 km s−1 is clearly required to fit the lines in their emission spectra (Brotherton et al., 1994; Mason et al., 1996; Crenshaw and Kraemer, 2007; Hu et al., 2008a,b; Zhu et al., 2009; Li et al., 2015). The open questions are: does the ILR exist physically separated from BLR and NLR? What are the mechanisms that give rise to ILR in some sources but not in others?

Recently, Adhikari et al., (2016 hereafter AD16) have shown, using the framework put forward by NL93, that when the density of illuminated clouds is high enough, the dust does not suppress the gap between BLR and NLR and intermediate line emission is clearly visible. The authors performed photoionisation simulations of radially distributed clouds subject to the radiation of four different spectral energy distributions (SEDs), most common types of AGN. The dust content was introduced at the sublimation radius of assumed value: Rd = 0.1 pc. In NL93, the authors assumed a power law with slope −1.5 and normalization 109.4 cm−3 at Rd to describe the variation of the density of gas clouds with distance from the nucleus. These clouds were then illuminated by the mean AGN spectrum. Resulting emissivity profiles contained the suppression of line emission between BLR and NLR as it is commonly observed. AD16 made one step forward, showing that if the density at the sublimation radius is high, of the order of 1011.5 cm−3 , the ILR is clearly visible. Such result appeared to be independent on SED of illuminated radiation taken into account. Additionally, the authors argued that the low ionization nuclear emission regions (LINERs) should also exhibit the ILR.

In this paper, we expand the work of AD16 and investigate the variation of density profile of radially distributed clouds. All photoionisation simulations are done with the most recent version of the CLOUDY code (Ferland et al., 2017). To accommodate broad types of ionizing SEDs in our calculations, we considered four distinct AGN types: Sy1.5 galaxy Mrk 509 (Kaastra et al., 2011), Sy1 galaxy NGC 5548 (Mehdipour et al., 2015), NLSy1 galaxy PMN J0948+0022 (D'Ammando et al., 2015), and LINER NGC 1097 (Nemmen et al., 2014), each of them obtained from currently available simultaneous multi-wavelength observations. As a result of our simulations, we derived the line emissivity radial profiles for major emission lines:[Hβ λ4861.36 Å, He II λ1640.00 Å, Mg II λ2798.0 Å, C III] λ1909.00 Å and [O III] λ5006.84 Å.

Adopting the density normalization to be 1011.5 cm−3 at Rd = 0.1 pc, all the power law density distribution yield continuous line emissivity profiles with prominent intermediate line emission component in permitted lines Hβ, He II, and Mg II, independent of the density slopes and the spectral radiation shapes adopted. Below we briefly outline the photoionisation model itself, and discuss the resulting line emissivity profiles focusing mainly on the visibility of ILR in different AGN.

## 2. PHOTOIONISATION MODEL

The simulation of the photoionisation process is done with the publicly available numerical code CLOUDY version c17 (Ferland et al., 2017), which takes into account all the relevant radiative processes when a gas cloud is subjected to an incident radiation field. A simplistic geometrical set up of gas distributed from BLR further out to NLR is arranged by assuming spherical clouds with varying gas density, nH ,and the total column density NH, at each radial distances, r, from the SMBH:

$$n\_{\rm H}(r) = 10^{11.5} \,\mathrm{(r/R\_d)}^{-\beta}, \qquad N\_{\rm H}(r) = 10^{23.4} \,\mathrm{(r/R\_d)}^{-1} \tag{1}$$

where β is the power law density slope, and Rd is arbitrarily chosen to be equal 0.1 pc (following NL93 and AD16). The total column density of a cloud located at the sublimation radius is assumed after NL93: NH (at Rd) = 1023.4 cm−2 , and the gas hydrogen density after AD16 : nH (at Rd) = 1011.5 cm−3 . Here, we stress that the density normalizations lower than the value adopted in this paper, do not reproduce the intermediate line emission as shown by AD16.

Besides three previously considered types of AGN incident radiation shapes: Sy1.5 galaxy Mrk 509 (Kaastra et al., 2011), Sy1 galaxy NGC 5548 (Mehdipour et al., 2015), NLSy1 PMN J0948+0022 (D'Ammando et al., 2015), in this paper we also used the shape appropriate for LINER NGC 1097 (Nemmen et al., 2014). This choice of SED covers the general shapes of the radiation emanating from the AGN central engine. Adopted SEDs are the incident spectra used in the photoionisation simulation, where clouds distributed along the range of radii are exposed to the same type of radiation. All the SEDs are normalized to the bolometric luminosity 1045 erg s−1 which is an input to the CLOUDY code. This allows to compute the ionizing flux i.e., ionization parameter at each cloud radius.

We adopted the CLOUDY default chemical abundances, which are mostly the Solar values derived by Grevesse and Sauval (1998) for the gas clouds at r ≤ Rd, whereas the interstellar medium (ISM) composition1 with dust grains is used for the clouds at r > Rd. This assumption is consistent with the observational suggestions that the BLR is devoid of dust whereas the lower temperature in NLR allows its existence. On moving further out from BLR to NLR, the increase in radial distance is accompanied by the decrease in ionization degree and a cloud thickness.

The aim of this paper is to search how the appearance of intermediate line emission is sensitive on the value of density power law slope. Below we present line luminosity radial profiles for six values of β = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0. We are aware that the power law density distribution of clouds does not reflect realistic situation in AGN, but it is sufficient for the purpose of this paper. In the forthcoming paper (Adhikari et al., in preparation), we plan to use realistic density profile, which is expected where clouds form from outflowing gas above the accretion disk atmosphere. Furthermore, we plan to consider measured values of bolometric luminosities, which give the realistic position of sublimation radius for each type of AGN due to the formula by Nenkova et al. (2008).

1 for details see the Hazy1 CLOUDY documentation.

FIGURE 1 | Left: Line emission vs. radius for Mrk 509 SED. Different subplots are for various density slopes given in the left corners. Major emission lines: Hβ (red circles), He II (green triangles down), [O III] (blue crosses), Mg II (cyan triangles up) and [C III] (magenta pluses) are shown for each density profile cases. For clarity total dust emission (magenta continuous line) and total gas emission (black dashed line) are also shown. Right: The same as in left but with the spectral radiation shape of NGC 5548.

# 3. LINE EMISSIVITIES

As the results of photoionisation simulations, we compute line luminosities emitted from clouds located at the each radii. Therefore, by presenting line luminosity dependence on the distance from SMBH, i.e., line luminosity radial profile, we can check if the emission from ILR is comparable to the BLR and NLR or substantially lower. This is our basic test for the existence of ILR in all considered types of AGN.

We derived the line luminosity radial profiles for the major emission lines : [Hβ λ4861.36 Å, He II λ1640.00 Å, Mg II λ2798.0 Å, C III] λ1909.00 Å, and [O III] λ5006.84 Å. The resulting line emissivity profiles for the four cases of SED are presented in the **Figures 1**, **2**. In all cases of density power law slopes, we recovered a continuous line emission, with a small enhancement of the permitted lines Hβ and He II at the radial distance around 0.1 pc corresponding to the intermediate region, independent of the shape of the SEDs in consideration. There is a small reduction of Mg II line at 0.1 pc though not very significant as compared to the suppression presented by NL93. The semi forbidden line C III contribution to the intermediate emission component becomes the most prominent for the density profile with β = 1.5. These results corroborate with the conclusion of AD16 that when the density of the emitting gas is high enough, the extinction effect of dust grains on line production is negligible.

The most noticeable effects of different density profiles on line emissivities occur in the NLR range, i.e., for r > 50 pc. This behavior is quite obvious since for those radii, differences in densities between profiles are the biggest. For β = 0.5 and 1.0, density falls slowly and remains moderately high across the radii causing the strong suppression of forbidden line [O III]. [O III] is effectively produced in low density environment and becomes prominent when the density around the radius 10 pc becomes low enough, the cases for the profiles with β ≥ 2.0. Narrow

line emission is dominated by the [C III] components when the density distribution is given by the profiles with β ≥ 2.0. We found that the derived line emissivities for all cases of power law density slopes, particularly in the region of intermediate line emission, do not strongly depend on the shapes of the SED used. There are subtle differences in emissivities corresponding to the BLR and NLR due to the different amount of UV and soft Xray photons among the SEDs. This result is in agreement with the conclusion of AD16, that the presence of ILR emission is not determined by the shape of the incident radiation.

#### 4. DISCUSSION

The results above confirm the conclusion of AD16, that the dust extinction of the emission lines in AGN introduced by NL93 is important only when the gas density is low. In AD16, the authors adopted only a density power law of slope β = 1.5 whereas this work demonstrates that the different slopes of the density distribution do not matter significantly as long as the gas density at the sublimation radius is high, in this case being 1011.5

cm−3 . In all cases of considered density profiles, we obtained an intermediate line emission around 0.1–1 pc, mostly manifested in permitted lines Hβ, He II, and Mg II, and weakly present in the semi forbidden line [C III]. This indicates that the high density and the low ionization environment favors the intermediate line emission rather than the high ionization environment where the forbidden line [O III] is produced. So, in the AGNs where the ILR is seen in observations, the emitting region is composed of the dense and less ionized gas.

The physical reason for the disappearance of the effect of dust is connected with the size of H- ionized front in the gas. At very high value of ionization parameter, i.e., for the low density case, the volume of the H-ionized region is very large, if not the full cloud volume. When the density of cloud increases, the ionization decreases, and a cloud consists of two regions: Hionized region and H-neutral region. The line emission comes from the H-ionized region, and only the dust in this region competes with the gas for the photons. In other words, the dense clouds have much smaller geometrical thickness of H -ionized layer, smaller dust column density in the region with abundant photons, and therefore the dust absorption is negligible. Our simulations are not yet aimed to make a quantitative statements about the studied objects. For that, we would need to do more extensive study, representing the bolometric luminosity and the position of the inner radius of the dust distribution appropriate for a given object. However, the grid of results shows a clear trend.

In the recent years, there has been promising claims that, broad line emission clouds in AGN are connected with the wind from the upper part of an accretion disk atmosphere (Gaskell, 2009; Czerny and Hryniewicz, 2011). As shown in the Fig. 6 of AD16, the density profiles computed in the upper part of the standard disk atmosphere can be quite dense with values up to ∼ 1015 cm−3 at the assumed position of sublimation radius. Those density values depend on the mass of the black hole and the disk accretion rate. Because of the high gas density, photoionisation simulations outcome with continuous line luminosity radial profile for the reason discussed in the previous paragraph. As the consequence, ILR should be observed together with BLR and NLR. The use of realistic density profiles expected from the accretion disk atmosphere is very important in

## REFERENCES

the aim to understand the nature and origin of the ILR observed in some AGNs. This work is in progress and will be presented by Adhikari et al. (in preparation).

## AUTHOR CONTRIBUTIONS

TPA, AR, and KH were responsible for developing the idea, doing the simulations, analyzing the results and writing the text for the manuscript, BC provided the concept and GF discussed the results.

#### FUNDING

This research was supported by Polish National Science Center grants No. 2016/21/N/ST9/03311, 2015/17/B/ST9/03422, 2015/18/M/ST9/00541, 2015/17/B/ST9/03436, and by Ministry of Science and Higher Education grant W30/7.PR/2013. It received funding from the European Union Seventh Framework Program (FP7/2007-2013) under the grant agreement No. 312789. TPA received funding from NCAC PAS grant for young researchers.

broad emission line region obscured by the dusty torus. Astrophys. J. 812:99. doi: 10.1088/0004-637X/812/2/99

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2017 Adhikari, Róza˙ nska, Hryniewicz, Czerny and Ferland. This ´ is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# EW[OIII] as an Orientation Indicator for Quasars: Implications for the Torus

Susanna Bisogni 1, 2 \*, Alessandro Marconi 2, 3, Guido Risaliti 2, 3 and Elisabeta Lusso4

1 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, United States, 2 Osservatorio Astrofisico di Arcetri, INAF, Florence, Italy, 3 Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Florence, Italy, 4 Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham, United Kingdom

We present an analysis of the average spectral properties of 12,000 SDSS quasars as a function of accretion disc inclination, as measured from the equivalent width of the [OIII] 5007Å line. The use of this indicator on a large sample of quasars from the SDSS DR7 has proven the presence of orientation effects on the features of UV/optical spectra, confirming the presence of outflows in the NLR gas and that the geometry of the BLR is disc-like. Relying on the goodness of this indicator, we are now using it to investigate other bands/components of AGN. Specifically, the study of the UV/optical/IR SED of the same sample provides information on the obscuring "torus." The SED shows a decrease of the IR fraction moving from face-on to edge-on sources, in agreement with models where the torus is co-axial with the accretion disc. Moreover, the fact we are able to observe the broad emission lines also in sources in an edge-on position, suggests that the torus is rather clumpy than smooth as in the Unified Model. The behavior of the SED as a function of EW[OIII] is in agreement with the predictions of the clumpy torus models as well.

Keywords: galaxies: active, galaxies: nuclei, galaxies: Seyfert, quasars: emission lines, quasars: general

# 1. INTRODUCTION

The fact we are not able to spatially resolve the inner regions of Active Galactic Nuclei (AGN), combined with their axisymmetric geometry (Antonucci and Miller, 1985; Antonucci, 1993), can make it difficult to interpret their emissions. The Unified Model predicts the orientation to be one of the main drivers of the diversification in quasars spectra. For this reason, an indicator of the inclination of the source with respect the line of sight of the observer is essential to get further in studying these objects.

Despite several quasars properties have been found to provide information on the inclination of the inner nucleus (Wills and Browne, 1986; Wills and Brotherton, 1995; Boroson, 2011; Decarli et al., 2011; Van Gorkom et al., 2015), we still lack an univocal measurement of this quantity. This problem is even harder when dealing with not-jetted objects, the most among quasars (>90%, Padovani, 2011), for which we can not rely on the presence of the strongly collimated radio-jets, directed perpendicularly to the accretion disc.

In order to give a more accurate description of the components surrounding the central engine and to understand where are the boundaries between one and another, we need to know which components are being intercepted by our line of sight.

#### Edited by:

Paola Marziani, Osservatorio Astronomico di Padova (INAF), Italy

#### Reviewed by:

Víctor Manuel Patiño Álvarez, National Institute of Astrophysics, Optics and Electronics, Mexico Luka C. Popovic, Belgrade Astronomical Observatory, Serbia

#### \*Correspondence:

Susanna Bisogni<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Milky Way and Galaxies, a section of the journal Frontiers in Astronomy and Space Sciences

Received: 25 September 2017 Accepted: 06 November 2017 Published: 21 November 2017

#### Citation:

Bisogni S, Marconi A, Risaliti G and Lusso E (2017) EW[OIII] as an Orientation Indicator for Quasars: Implications for the Torus. Front. Astron. Space Sci. 4:48. doi: 10.3389/fspas.2017.00048

Assuming that some of these inner components are characterized by a spherical geometry can often simplify the scenario, while at the same time misleading us. We use the emission lines coming from the Broad Line Region (BLR) to give an estimate of the mass of the central Super Massive Black Hole (SMBH), but in doing that we do not take properly into consideration the geometry of the BLR, i.e., we use an average virial factor f to account for the uknown in the geometry and kinematics of the emitting region and we overlook the effects of orientation on the emission lines (Jarvis and McLure, 2006; Shen, 2013). These measurements can then be used in turn to examine the relations between the SMBH and their host galaxies—one of the few tools available to understand the connection between structures on such different spatial scales—their uncertainties affecting these studies (e.g., Shen and Kelly, 2010). If the BLR is characterized by a non-spherical geometry we are sistematically underestimating the BH masses in all the sources in which the velocity we intercept, i.e., the line width we measure, is only a fraction of the intrinsic velocity of the emitting gas orbiting around the SMBH. The inclination of the source with respect to the line of sight is therefore crucial to both the understanding of how the nuclei work and how they affect the formation and evolution of galaxies in the Universe. In this proceeding we show recent results on the optical spectra and we present a preliminary result on the Spectral Energy Distribution of quasars, that we obtained using the EW[OIII] as an orientation indicator.

# 2. ORIENTATION EFFECTS ON EMISSION FEATURES

#### 2.1. Optical Spectra

Based on the properties of the [OIII] 5007Å line—negligibly contaminated by non-AGN processes and coming from the Narrow Line Region (NLR), whose dimensions ensure the isotropy of the emissions (Mulchaey et al., 1994; Heckman et al., 2004)—and on the strong anisotropy of the continuum emitted by the optically-thick/geometrically-thin accretion disc (Shakura and Sunyaev, 1973), we proposed the equivalent width (EW) of the [OIII] line, the ratio between the two luminosities, as an indicator of quasars orientation (Risaliti et al., 2011; Bisogni et al., 2017).

In Risaliti et al. (2011) we examined the distribution of the observed EW[OIII] in a large sample of quasars from the SDSS DR5 (∼ 6, 000) and verified the presence of an orientation effect: the distribution shows a power law tail at the high EW[OIII] values that can not be ascribable to the intrinsic differences in the NLR among different objects, i.e., the intrinsic EW[OIII] distribution, the one we would observe if all the sources were seen in a face-on position. The observed EW[OIII] distribution is a convolution of the intrinsic properties of the NLR emissions in the different objects, such as the ionising continuum and the covering factor of the clouds, and the effects due to their inclination angle.

In Bisogni et al. (2017) we selected a larger sample of objects from the SDSS DR7 (∼12,000), this time with the aim of looking for evidences of orientation effects in the optical spectra. We split our sample in six bins of EW[OIII], each one corresponding to an inclination angle range. Within each bin, the spectra were stacked in order to produce a master spectrum. We then analysed both the broad and the narrow emission lines as a function of EW[OIII], i.e., of the inclination angle, finding orientation effects on both of them. **Figure 1** shows the presence of orientation effects on the broad component of Hβ: the width of the broad line, either represented by the line dispersion σ, the Full Width Half Maximum (FWHM) or the Inter-Percentile Velocity width (IPV), increases steadily when we move from low to high EW[OIII]. We found the same result for the other broad lines examined (Hα and MgII, see Bisogni et al., 2017 for more details). This behavior is what is expected if the BLR geometry is disc-like and we are moving from sources in a face-on position to sources in an edge-on position.

As for the narrow emission lines, we examined the [OIII] λ5007Å, the most prominent among them in the optical spectrum. This line is known to be contaminated by emissions coming from non-virialized gas, i.e., not orbiting around the central SMBH, but outflowing perpendicularly to the accretion disc (Heckman et al., 1984; Boroson, 2005). If the EW[OIII] is a good indicator of the inclination of the source, we should see the blue component of the line, emitted by outflowing gas, decreasing both in intensity and in velocity shift with respect to the nominal wavelength of the emission moving to high EW[OIII] values. Going from face-on to edge-on position in fact we are not intercepting anymore the outflow perpendicular to the accretion disc. This behavior is found in the [OIII] line profile (**Figure 2**).

We want to stress that within each EW[OIII] bin, therefore within each inclination angle range, the population of quasars is characterized by different SMBH masses, luminosities and accretion rates. These properties are considered among the main drivers of the variance in quasars spectra (Zamanov and Marziani, 2002; Marziani et al., 2003; Shen and Ho, 2014). In our study however, as it is designed, even if the effects produced by these properties are present, they are diluited in the stacked representative spectra. As a confirmation that the orientation, even if not the only driver, plays a major role in the variance of quasars spectra, we found a clear trend of the Eigenvector 1, i.e., the anticorrelation between the FeII and [OIII] emissions intensity that Boroson and Green (1992) identify as the main responsible for quasars spectral variance, with the EW[OIII]. Specifically, when we move from low to high EW[OIII], i.e., from low to high inclination angles, we see the [OIII] intensity increasing, while FeII emission becomes less and less intense. This can be explained in terms of orientation: the BLR shares the same anisotropy of the accretion disc and therefore the intensity of its emissions, in this case FeII, is decreased by a factor cos θ, i.e., decreases when moving to edge-on sources. On the other hand the [OIII] line appears as more evident in edge on positions because the luminosity of the continuum emitted by the accretion disc is decreased by the factor cos θ.

## 2.2. Infrared Emissions

The observed EW[OIII] distribution has implications for the obscuring component as well.

FIGURE 1 | Full Width Half Maximum, Inter-Percentile Velocity width and dispersion σ as a function of the EW[OIII] for the Hβ line. All these quantities, describing the rotational velocity of the gas orbiting around the central SMBH, increase moving from low to high EW[OIII] as expected if the BLR is disc-shaped and we are moving from face-on to edge-on positions.

component of the gas is not intercepted anymore by the line of sight.

The torus is depicted in the Unified Model as a smooth and toroidal structure that can reach ∼1–10 pc in size (Burtscher et al., 2013). If this is true then, there is a maximum inclination angle beyond which we are not able anymore to intercept the emissions coming from the very inner components, such as the continuum emitted by the accretion disc and the broad lines emitted by the BLR. In this case, however, the observed EW[OIII] distribution would drop when the line of sight is starting to intercept the torus. This is not what we observe: the power law keeps going very steadily to the highest EW[OIII] values. Moreover, we are intercepting broad emission lines in positions corresponding to high inclination angles. Both these facts are not compatible with the torus being a smooth structure and rather suggest a clumpy structure.

To test the indicator and exploit its potential, we are now interested in investigating the infrared emissions.

We then collect photometric data for the same sample in the UV, optical and IR band to study the Spectral Energy Distributions (SED) of the sources.

# 3. SAMPLE AND DATA ANALYSIS

For the sample of ∼12,000 objects we selected from the SDSS DR7 the following photometric data are available:

We first correct all the magnitudes for Galactic extinction using the maps from the Schlegel et al. (1998). Then, for each EW[OIII] bin, we use the same approach as for the optical spectroscopic data: we rest-frame the data according to the sources redshift and then we perform a stacking of the interpolated SED in order to produce a master SED, on which we can examine the effects produced by the orientation. Before stacking them, we normalize each SED by dividing for the value of νLν at λ = 15µm, a reference wavelength in the mid-infrared spectral range, the band in which the torus emits. In doing that, we are normalizing the individual SED for the intrinsic differences of the torus and of other components in the different objects, such as the size and covering factor of the obscuring region, its distance from the central engine and the properties of the ionising continuum, whose emission is being reprocessed by the torus. This makes us able to compare the average behavior of the obscuring structure at different inclinations with respect to the line of sight of the observer. The final SED are shown in **Figure 3**.

# 4. RESULTS AND DISCUSSION: IMPLICATIONS FOR THE OBSCURING TORUS

In the optical stacks corresponding to the highest inclination angles (high EW[OIII]) we are able to detect emissions from the BLR. This evidence implies three possible scenarios: the absence of the torus, a torus that is mis-aligned with respect the plane of the accretion disc (and of the BLR) and a clumpy torus. The first scenario is ruled out by the fact that the IR emission is clearly visible in the SED of the sample (**Figure 3**). As for the second one, we see the IR emission in the stacked SED decreasing progressively as a function of the indicator, defined through the anisotropic properties of the emission coming from the accretion disc itself. If torus and accretion disc were not co-axial, we would not see such an orderly behavior.

The only scenario we are left with is therefore a clumpy torus, leading to a differention between type 1 and type 2 AGN due only to the photon escaping probability (Elitzur, 2008). Due to the selection we performed (i.e., we selected blue objects, and verified that the continuum in our stacked optical spectra was not experiencing any reddening, see Bisogni et al., 2017), when we are looking at sources with a high EW[OIII], i.e., with a high inclination angle, we are dealing with type 1 sources in which the BLR is intercepted through the dusty clouds of the torus.

We can compare our results with the clumpy models in literature (Nenkova et al., 2008a,b) that examine the infrared emission of the torus as a function of the inclination angle with respect to the observer. If the torus is a clumpy structure, what we expect is that the IR emission at shorter wavelengths decreases progressively more than the ones at longer wavelengths when we are reaching edge-on position, due to the combination of an increasing number of clouds intercepted by the line of sight and of a higher absorption at the shorter than at the longer wavelengths (Nenkova et al., 2008b).

The behavior of the stacked SED as a function of EW[OIII] confirms this scenario. At low EW[OIII] (low inclination angle) we are able to intercept the IR emissions coming from the inner clouds of the torus, that are directly illuminated by the ionizing continuum, while at high EW[OIII] (high inclination angle) the IR emission coming from the inner clouds is shielded and we can detect it only after it is absorbed by the clouds in the outskirts of the torus. This produces the decrease in the flux at the shorter wavelengths, that becomes progressively more important for stacks corresponding to higher inclinations.

FIGURE 3 | Left: Spectral Energy Distributions for the six EW[OIII] bins, corresponding to different inclination angle ranges. The master SED for a EW[OIII] bin was realized as follows: the photometric data for each source from the GALEX, SDSS, 2MASS and WISE surveys were corrected for Galactic reddening, rest-framed, interpolated on a common grid and normalized to the νLν value corresponding to the reference wavelength (15µm); for every channel in the grid we then selected the median value. Right: Total flux for the six EW[OIII] bins in the spectral range in which the emission coming from the torus is predominant. SED corresponding to low EW[OIII] values are characterized by a shallower decrease in the emission at the shorter IR wavelengths with respect to the longer ones, while in the case of high EW[OIII] the decrease is steeper. This behavior is in agreement with the clumpy torus models (see text for details).

As a final verification that our results are not biased by any characteristics of the sample, we made the following checks:

#### 5. CONCLUSIONS

In this proceeding we summarize the results of the analysis on the optical spectra and we present the preliminary results of the analysis on the infrared emission of 12, 000 sources of the SDSS DR7 as a function of the EW[OIII], a new orientation indicator. We find that:

## REFERENCES

the shift with respect to the reference wavelength moving from face-on to edge-on positions.

Further analysis is needed in order to investigate properly the emission coming from the torus. Starting from these first results, we are in the process of performing a SED fitting for each source in the sample with AGNfitter (Calistro Rivera et al., 2016). We will then be able to repeat the analysis on the representative SED, this time having also information on the single components contributing to the total emission.

We will also investigate the sources in our sample for which multiple observations are available (e.g., Stripe82, new BOSS spectra) in order to look for evidences of changing look behaviors as a function of the EW[OIII]. If our interpretation of the data implying a clumpy structure for the torus is correct, we expect to see some of the sources that were included in our sample as Type 1 AGN changing to Type 2 objects at a different epoch. This behavior is expected more frequently for sources with a high EW[OIII], where the orientation effect is dominant, but it is not excluded even for sources with low EW[OIII] values.

## AUTHOR CONTRIBUTIONS

All the authors contributed to the work and approved it for publication.

### FUNDING

Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Number AR7-18013 X issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060 (10.13039/100000104). EL is supported by a European Union COFUND/Durham Junior Research Fellowship (under EU grant agreement no. 609412).

Boroson, T. (2005). Blueshifted [OIII] emission: indications of a dynamic narrowline region. Astron. J. 130, 381–386. doi: 10.1086/431722

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2017 Bisogni, Marconi, Risaliti and Lusso. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# New Constraints on Quasar Broad Absorption and Emission Line Regions from Gravitational Microlensing

Damien Hutsemékers 1 \*, Lorraine Braibant 1 , Dominique Sluse1 , Timo Anguita2 and René Goosmann3

1 Institut d'Astrophysique et de Géophysique, Université de Liège, Liège, Belgium, 2 Departamento de Ciencias Fisicas, Universidad Andres Bello, Santiago, Chile, 3 Observatoire Astronomique de Strasbourg, Université de Strasbourg, Strasbourg, France

Gravitational microlensing is a powerful tool allowing one to probe the structure of quasars on sub-parsec scale. We report recent results, focusing on the broad absorption and emission line regions. In particular microlensing reveals the intrinsic absorption hidden in the P Cygni-type line profiles observed in the broad absorption line quasar H1413+117, as well as the existence of an extended continuum source. In addition, polarization microlensing provides constraints on the scattering region. In the quasar Q2237+030, microlensing differently distorts the Hα and CIV broad emission line profiles, indicating that the low- and high-ionization broad emission lines must originate from regions with distinct kinematical properties. We also present simulations of the effect of microlensing on line profiles considering simple but representative models of the broad emission line region. Comparison of observations to simulations allows us to conclude that the Hα emitting region in Q2237+030 is best represented by a Keplerian disk.

#### Edited by:

Mauro D'Onofrio, Università degli Studi di Padova, Italy

#### Reviewed by:

Giovanna Maria Stirpe, Osservatorio Astronomico di Bologna (INAF), Italy Milan S. Dimitrijevic, Astronomical Observatory, Serbia

#### \*Correspondence:

Damien Hutsemékers<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Milky Way and Galaxies, a section of the journal Frontiers in Astronomy and Space Sciences

Received: 23 August 2017 Accepted: 19 September 2017 Published: 29 September 2017

#### Citation:

Hutsemékers D, Braibant L, Sluse D, Anguita T and Goosmann R (2017) New Constraints on Quasar Broad Absorption and Emission Line Regions from Gravitational Microlensing. Front. Astron. Space Sci. 4:18. doi: 10.3389/fspas.2017.00018 Keywords: quasars, broad emission lines, broad absorption lines, gravitational microlensing, polarization

## 1. INTRODUCTION

When the light from a distant quasar passes through the gravitational field of a galaxy, it is deflected and multiple magnified images of the source are observed. In addition, stars in the lensing galaxy can act as microlenses and produce an extra magnification of some images. The collective effect of these stars generates a complex magnification pattern in the source plane that takes the form of a caustic network. Microlensing magnification varies in time due to the relative motions of the source, lens and observer, on timescales of weeks to years. High magnification events occur when caustics are close to the line of sight (e.g., Schmidt and Wambsganss, 2010, for a review).

Gravitational lensing magnification strongly depends on the Einstein radius of the system, in the sense that only sources smaller than this radius can be significantly magnified. For a typical lensed system with a lensing galaxy at redshift z ∼ 0.5 and a source quasar at z ∼ 2, the projected Einstein radius of an average 0.3 M⊙ star in the lensing galaxy is of the order of 10−3 pc, which is comparable to the size of the quasar continuum-emitting accretion disk. Microlensing thus mainly magnifies the continuum source. The more extended broad emission line region is either unaffected or only partly magnified. The much larger narrow line region is totally unaffected.

Therefore, microlensing differently magnifies the various components of the quasar spectrum. By comparing the spectra of two images of a lensed quasar, one affected by microlensing and the other not, one can separate the part of the spectrum which is microlensed, that is the part coming from the most compact source, from the part of the spectrum which is not affected by microlensing and originates from a more extended region. Information on the geometry and kinematics of the quasar inner regions can thus be obtained. This is illustrated in the following sections, focusing on the broad absorption and emission line regions (respectively, BALR and BELR).

## 2.  Image A is not or weakly affected by microlensing, thus providing a reference spectrum. We can then separate the part of the line profiles that is microlensed from the part that is not microlensed using the macro-micro decomposition (MmD) method presented in Sluse et al. (2007) and Hutsemékers et al. (2010).

**Figure 1** illustrates the decomposition of the CIV λ1549Å P Cygni-type line profile into its microlensed (FMµ) and nonmicrolensed (FM) components (see Hutsemékers et al., 2010; Sluse et al., 2015, for more details). FMµ comes from a compact region that essentially produces the absorbed continuum, while FM comes from a more extended region too large to be microlensed. From this spectral decomposition, it results that (1) the BAL profile consists of a completely black absorption (seen in

FIGURE 1 | Microlensing in the CIV line of the BAL quasar H1413+117. FA is the observed spectrum of image A which is not affected by microlensing. The line profile can be decomposed in a part affected by microlensing, FMµ, and another part not affected by microlensing, FM, with FA = FM + FMµ.

FMµ) partially filled in by the broad emission line (seen in FM); (2) this absorption does not start at zero-velocity but at an onset velocity of ∼2,000 km/s; (3) the broad emission line itself is reabsorbed (see FM) over a wavelength range narrower than the full absorption profile, revealing the existence of an additional, more extended BALR; (4) a part of the continuum (seen in FM) is not microlensed thus originating from a region larger than the source of the continuum seen in FMµ. These observations suggest a two-component outflow: one component is co-spatial with the BELR, while the other one, more distant, partially re-absorbs the emission from the BELR (Borguet and Hutsemékers, 2010). O'Dowd et al. (2015) found in addition an ionization dependence of the BAL onset velocity that they interpreted in the framework of a disk-wind model.

The light from H1413+117 is linearly polarized and Chae et al. (2001) found evidence for polarization microlensing. Hutsemékers et al. (2015) obtained spectropolarimetric observations of the different images that suggest that the continuum is scattered off two regions generating roughly orthogonal polarizations: a compact region located in the equatorial plane close to the accretion disk, which is microlensed, and an extended region located along the polar axis, which is not microlensed. This indicates that the non-microlensed extended continuum seen in FM (**Figure 1**) could originate from scattering.

# 3. MICROLENSING OF THE BROAD EMISSION LINE REGION IN Q2237+030

### 3.1. Observed Line Profile Distortions

Though larger than the source of continuum, the BELR can be partly magnified by microlensing, which results in line profile deformations.  By selectively magnifying different subregions of the broad line region, gravitational microlensing can thus provide information on the size, geometry and kinematics of the BELR in quasars (Nemiroff, 1988; Schneider and Wambsganss, 1990). In particular, the size of the BELR has been estimated in a few objects and found compatible with reverberation mapping measurements (Wayth et al., 2005; Sluse et al., 2011; Guerras et al., 2013).

Making use of the MmD line profile disentangling technique, Braibant et al. (2014, 2016) have extracted the part of the emission line profiles that is affected by microlensing in two lensed quasars for which high quality data were available. **Figure 2** illustrates the FMµ / FM decomposition of the CIV λ1549Å and Hα broad emission line profiles for the quadruply lensed quasar Q2237+030, the Einstein Cross, at an epoch when microlensing was prominent in image A and negligible in image D. A clear difference in the distortions suffered by the CIV and Hα emission

FIGURE 2 | Microlensing in the CIV and Hα lines of the quasar Q2237+030. FD is the observed spectrum of image D which is not affected by microlensing. The line profile can be decomposed in a part affected by microlensing, FMµ, and another part not affected by microlensing, FM, with FD = FM + FMµ. The dotted line profile represents the continuum-subtracted FMµ.

lines can be noticed. In CIV the effect is essentially symmetric, the wings of the line being more magnified than the core, while in Hα the effect is asymmetric, the red wing being more magnified than the blue wing. Since these observations were obtained at the same epoch, both the CIV and Hα emitting regions are magnified by the same caustic pattern. This indicates that the high- and low-ionization emission lines must originate from regions with distinct kinematical properties.

As illustrated in **Figure 3**, the detection of a red/blue differential microlensing effect in the Hα line profile suggests that the low-ionization BELR is likely a Keplerian disk, while the wings/core distortion observed in the CIV line can be interpreted assuming a polar outflow model. However, the mapping between a wavelength range in the line profile and subregions of the BELR seen in projection is usually not unique and a confrontation of the observations to detailed modeling appeared necessary.

#### 3.2. Simulations of Line Profile Distortions

Possible effects of microlensing on broad emission lines have been theoretically investigated by several authors (Nemiroff, 1988; Schneider and Wambsganss, 1990; Hutsemékers et al., 1994; Popovic et al., 2001; Abajas et al., 2002, 2007; Lewis and ´ Ibata, 2004; O'Dowd et al., 2011; Simic and Popovi ´ c, 2014 ´ ) considering various BELR models and magnification patterns.

In Braibant et al. (2017), we extend those works focusing on wings/core and red/blue line profile distortions in order to constrain the BELR models. The effects of gravitational microlensing on the quasar broad emission line profiles and their underlying continuum have been simultaneously computed, considering simple, representative BELR, accretion disk, and magnifying caustic models. Keplerian disks as well as polar and equatorial outflow models of various sizes have been considered. The effect of microlensing has been quantified using four observables: µ BLR, the total magnification of the broad emission line, µ cont, the magnification of the underlying continuum, as well as red/blue, RBI, and wings/core, WCI, indices that characterize the line profile distortions. Those quantities were designed to not depend on the exact profile of the broad emission lines, so that they can be directly compared to observations.

The simulations show that asymmetric distortions of broad line profiles like those reported in Braibant et al. (2014, 2016) can be reproduced, and attributed to the differential effect of microlensing on spatially and kinematically separated regions of the BELR. In particular, red/blue asymmetric distortions constitute a good discriminant between the polar outflow and other models. We then built diagrams that can serve as diagnostic tools to discriminate between the different BELR models making use of quantitative measurements of the four observables µ cont , µ BLR , RBI, WCI. It appeared from the simulations that only strong microlensing effects can produce distinctive line profile distortions for a limited number of caustic configurations, i.e., distortions that allow us to put serious constraints on the various BELR models.

The four indices µ cont , µ BLR , RBI, WCI were then measured for both the CIV and Hα emission lines observed in the lensed quasar Q2237+030, and compared to the values generated by the simulations (Braibant et al., in preparation). In the simulations, we assumed that the CIV and Hα BELRs, and the continuumemitting accretion disk share the same inclination with respect to the line of sight and the same location on the caustic network. From this comparison it results that the Hα lowionization BELR is best represented by a Keplerian disk while the CIV high-ionization BELR can be either a Keplerian disk

FIGURE 3 | Microlensing of different BELR structures and the resulting line profile distortions. The left panel sketches two models: a rotating Keplerian disk and a biconical outflow, both seen at intermediate inclination. The location of the approaching and receding gas is indicated in blue and red, respectively. The middle panel shows the BELR superimposed on a typical caustic. At the different positions indicated by colored losanges, different subregions of the BELR are magnified by the caustic. The right panel illustrates the line profiles corresponding to the different positions of the BELR onto the caustic pattern. Microlensing of the Keplerian disk is characterized by asymmetric red/blue line profile distortions while microlensing of the biconical outflow is characterized by symmetric wings/core distortions.

or a polar wind. Examples of fitting models are illustrated in **Figure 4**. In all cases the Hα BELR is roughly a factor 2 larger than the CIV BELR. Recent results from velocity-resolved reverberation mapping also suggest that the low-ionization BELR can be a Keplerian disk (Grier et al., 2017, and references therein).

# 4. CONCLUSIONS

These results demonstrate the potential of microlensing to probe the geometry and kinematics of the broad line regions and outflows in quasars. Constraints on the location of the scattering regions at the origin of the polarization can also be obtained. Simulations of line profile distortions show that only strong microlensing effects can produce line profile distortions allowing us to discriminate between various BELR models using single epoch data. To benefit from weaker microlensing effects, statistical analyses of larger data sets would be needed. In particular, a long-term spectrophotometric monitoring of the different images of lensed quasars would provide a real scan of their line emitting regions, with the possibility to constrain more complex models than those considered here. Microlensing can thus be a powerful and alternative approach to reverberation mapping, especially since it can be applied to high redshift quasars, with little dependence on their

#### REFERENCES

luminosity, and to the study of both the low- and high-ionization BLRs.

#### AUTHOR CONTRIBUTIONS

DH and DS initiated the project. LB, DH, DS, and TA contributed to the observations, data reduction and analysis. LB, DH, DS, and RG contributed to the BELR microlensing modeling. All authors contributed to the discussions. DH wrote the paper with contributions from DS. LB made the artwork in **Figure 3**.

#### ACKNOWLEDGMENTS

DH and LB acknowledge support of Belgian F.R.S.-FNRS. Support for TA is provided by project FONDECYT 11130630 and the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2017 Hutsemékers, Braibant, Sluse, Anguita and Goosmann. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Disentangling Accretion Disk and Dust Emissions in the Infrared Spectrum of Type 1 AGN

Antonio Hernán-Caballero1, 2 \*, Evanthia Hatziminaoglou2 , Almudena Alonso-Herrero3 and Silvia Mateos 4

1 Departamento de Astrofísica y CC. de la Atmósfera, Facultad de CC. Físicas, Universidad Complutense de Madrid, Madrid, Spain, 2 European Southern Observatory, Garching bei München, Germany, 3 Centro de Astrobiología (CSIC-INTA), Madrid, Spain, 4 Instituto de Física de Cantabria (CSIC-UC), Santander, Spain

We use a semi-empirical model to reproduce the 0.1–10µm spectral energy distribution (SED) of a sample of 85 luminous quasars. In the model, the continuum emission from the accretion disk as well as the nebular lines are represented by a single empirical template (disk), where differences in the optical spectral index are reproduced by varying the amount of extinction. The near- and mid-infrared emission of the AGN-heated dust is modeled as the combination of two black-bodies (dust). The model fitting shows that the disk and dust components are remarkably uniform among individual quasars, with differences in the observed SED largely accounted for by varying levels of obscuration in the disk as well as differences in the relative luminosity of the disk and dust components. By combining the disk-subtracted SEDs of the 85 quasars, we generate a template for the 1–10µm emission of the AGN-heated dust. Additionally, we use a sample of local Seyfert 1 galaxies with full spectroscopic coverage in the 0.37–39 µm range to demonstrate a method for stitching together spectral segments obtained with different PSF and extraction apertures. We show that the disk and dust templates obtained from luminous quasars also reproduce the optical-to-mid-infrared spectra of local Seyfert 1s when the contribution from the host galaxy is properly subtracted.

Keywords: galaxies:active, galaxies:Seyfert, quasars:general, infrared:galaxies, methods:data analysis, techniques:spectroscopic

# 1. OBTAINING ACCRETION DISK AND DUST TEMPLATES FROM LUMINOUS QUASARS

We use a sample of 85 luminous quasars (νLν [3µm]>1045.5 erg s−1 ) selected for their spectroscopic coverage (AKARI and/or Spitzer/IRS) in the rest-frame 2.5–10µm range. In addition to the AKARI and Spitzer spectroscopy we obtain optical photometry from the Sloan Digital Sky Survey (SDSS) Data Release 12, near-infrared (NIR) photometry from the Two Micron All Sky Survey (2MASS), the VISTA Hemisphere Survey (VHS), and the UKIRT Infrared Deep Sky Survey (UKIDSS), and mid-infrared photometry in 4 bands from WISE (see Hernán-Caballero et al., 2016, for details).

In such luminous quasars the optical emission of the AGN easily outshines that of the host galaxy. The emission from dust is expected to be negligible at λ . 0.85µm because the maximum temperature of dust grains is limited by sublimation to ∼1,500 K (Granato and Danese, 1994). As a

#### Edited by:

Ascensión Del Olmo, Consejo Superior de Investigaciones Científicas (CSIC), Spain

#### Reviewed by:

Isabel Marquez Perez, Instituto de Astrofísica de Andalucía (CSIC), Spain Øyvind Geelmuyden Grøn, Oslo and Akershus University College, Norway

> \*Correspondence: Antonio Hernán-Caballero

#<EMAIL_ADDRESS>
Specialty section: This article was submitted to Cosmology, a section of the journal Frontiers in Astronomy and Space Sciences

Received: 23 August 2017 Accepted: 11 October 2017 Published: 31 October 2017

#### Citation:

Hernán-Caballero A, Hatziminaoglou E, Alonso-Herrero A and Mateos S (2017) Disentangling Accretion Disk and Dust Emissions in the Infrared Spectrum of Type 1 AGN. Front. Astron. Space Sci. 4:30. doi: 10.3389/fspas.2017.00030 consequence, only the accretion disk and the emission lines from the broad line region (BLR) and narrow line region (NLR) contribute significantly to the rest-frame 0.1–0.85µm spectrum.

In Hernán-Caballero et al. (2016) we showed that a single empirical quasar template like that of Shen (2016) suffices to model the rest-frame UV-optical (0.1–0.85µm) SED of luminous quasars if we allow for an adjustable extinction to reproduce the diversity in optical spectral indices among individual quasars. To extend the template into the NIR and MIR ranges, we assumed that the disk emission follows a power-law with the theoretical slope α = 1/3 predicted for a locally heated optically thick disk (e.g., Shakura and Sunyaev, 1973; Hubeny et al., 2001) and confirmed through polarized light observations (Kishimoto et al., 2008). We also added to the new template the NIR nebular lines extracted from the quasar template of Glikman et al. (2006).

We fit the rest-frame 0.1–10µm SED of the 85 quasars with a two component disk+dust model. The disk component (plus nebular lines) is represented by the template described above, modified by an adjustable level of extinction with a wavelength dependency following the Small Magellanic Cloud Bar extinction law (Gordon et al., 2003). This law is often used to de-redden quasars (e.g., Hopkins et al., 2004; Glikman et al., 2012) since it lacks the 2,175 Å absorption feature. The AGN-heated dust component is represented by the linear combination of two black-bodies (hot and warm) at adjustable temperatures within the intervals 850–2,000 K and 150–900 K, respectively.

**Figure 1** shows examples of the best-fitting disk+dust decomposition model for a representative sub-sample of the 85 quasars. The residuals of the fit around ∼10% are consistent with the photometric uncertainties. We find a systematic excess emission in the 1.0–1.5µm range relative to the model. The median, mean, and standard deviation of the excess at restframe 1.2µm is 32, 40, and 33%, respectively. This is comparable to the values found in Hernán-Caballero et al. (2016) in spite of the stronger NIR continuum and inclusion of NIR nebular lines in the new disk template, supporting our previous claim that the excess originates in the dust component.

The distributions of AV for the extinction of the accretion disk and the relative luminosity of the dust and disk components (represented by the luminosity ratio between λ = 3.0µm and λ = 0.5µm) are shown in **Figure 2**. Negative values of AV are needed to model the disk of the bluest ∼25% of quasars because the disk template is an average of observed spectra that have not been de-reddened. Therefore the extinction values obtained in the fit are relative to the (unknown) average extinction of the sample in Shen (2016). Moderate values of AV (between −0.2 and 0.2 mag) fit all but the reddest 15% of quasars. The νLν (3µm)/νLν (0.5µm) ratio peaks at ∼1, indicating that the peak luminosities of the disk and dust components are typically similar, albeit the full range of variation being a whole order of magnitude.

**Figure 3** shows the infrared SEDs of the individual quasars, normalized at rest-frame 3µm, after subtraction of the disk component. The SEDs show little dispersion (<0.3 dex) between ∼1.5 and ∼6µm, suggesting a largely invariable spectrum for the hot dust, that is consistent with black-body emission at a temperature close to the grain sublimation limit. The dispersion increases at longer wavelengths at least in part due to the

FIGURE 1 | Spectral energy distributions and their best fitting disk+dust models for a representative subsample of the luminous quasar sample showing a range of AV and dust-to-disk luminosity ratios. Circles, stars, and squares represent, respectively, broadband photometry in the observed-frame optical (from SDSS), NIR (2MASS/UKIDSS/VHS), and MIR (WISE). Open symbols indicate bands outside the wavelength range used to fit the disk or dust components. The disk model is shown as a green dot-dashed line. The (AKARI+)IRS spectrum resampled at 1λ/λ = 0.05 is shown with small black dots. Gray dots below the (AKARI+)IRS spectrum represent the dust spectrum obtained after subtraction of the disk component. The model for the AGN-heated dust emission (red dashed line) is the linear combination of two black-bodies at adjustable temperatures (blue dotted lines). The combined disk+dust model is represented by the solid black line. The vertical dotted lines indicate the rest-frame wavelength of the recombination lines Hα and Paα.

onset of the broad silicate feature (which may be in emission or absorption). On the other hand, the larger dispersion at λ . 1.5µm is a consequence of the uncertainty introduced by the subtraction of a disk component that is increasingly dominant at shorter wavelengths.

We obtain a template for the AGN-heated dust by averaging the AKARI+IRS spectra of the individual quasars. At λ < 2µm only a few quasars have spectroscopic coverage, therefore we rely on the disk-subtracted broadband photometry to extend the template to λ < 2µm. We find that a 1,400 K black-body is consistent with most of the broadband data-points.

#### 2. APPLICATION TO LOCAL SEYFERT 1 GALAXIES

To test whether the templates for disk and dust emission obtained for luminous quasars are also representative of the nuclear emission in less luminous type 1 AGN, we have performed spectral decomposition of a sample of 13 local (z < 0.07) Seyferts and quasars with rest-frame 3µm luminosities in the range 1042.8−44.4 erg s−1 . The sample is a subset of the 23 broademission line AGN observed by Landt et al. (2008). We chose this sample because they obtained nearly simultaneous spectra in the optical (0.37–0.75µm) and NIR (0.8–2.4µm) with the FAST and SpeX spectrographs on the Tillinghast 1.5 m telescope and the NASA Infrared Telescope Facility, respectively. Out of their 23 sources, we select only the 13 that also have AKARI 2.5–5.0µm spectra from (Kim et al., 2015) and Spitzer/IRS 5.2– 39µm spectra in the CASSIS (Lebouteiller et al., 2011, 2015) or ATLAS-IRS (Hernán-Caballero and Hatziminaoglou, 2011) databases. Therefore we have continuous spectroscopic coverage from ∼0.35 to ∼35µm in the restframe. However, since our templates are not defined beyond 10µm, we perform the spectral decomposition only in the ∼0.35–10µm range.

Spectral decomposition on local type 1 AGN is more challenging compared to luminous quasars because the emission from the host galaxy is no longer negligible. The contribution from the stars to the total flux is important only at optical and NIR wavelengths, while the emission of the interstellar medium (ISM), in particular dust grains and aromatic hydrocarbons, is significant only in the mid-infrared. Since the integrated spectrum of the stars in a galaxy peaks at 1.6µm and its NIR shape does not vary substantially among the different spectral types, we can use a single template to represent the stellar emission of the host galaxy in all the sources. We choose the S0 galaxy template from Polletta et al. (2007). In the optical, the actual spectra of the host galaxies may diverge from the template at shorter wavelengths depending on the age and extinction of the stellar population, but the difference only becomes important at λ . 0.4µm.

Another difficulty in decomposing local galaxies is that while the AGN emission is always spatially unresolved, the host galaxy is extended and its contribution to the observed spectrum depends on the spatial resolution of the observations and the size of the extraction aperture (see Hernán-Caballero et al., 2015 for a discussion). The extraction aperture of the slit-less AKARI observations (60′′ × 7.5′′) is wider than the IRS slit (3.6′′ for SL1 and SL2 modules), and both are larger than those of the FAST (3′′) and SpeX (0.8′′) observations. To compensate for this, we compute the best-fitting disk+dust+stellar model for the local AGN as follows: for each of the four spectral segments (FAST, SpeX, AKARI, IRS) we obtain the values of the parameters that minimize the residuals while fitting by least squares the model:

$$F(\lambda) = af\_{disk}(\lambda)e^{-A\_V \tau(\lambda)} + bf\_{dust}(\lambda) + cf\_{star}(\lambda) \tag{1}$$

FIGURE 4 | Spectral decompositions of local type 1 AGN. The gray thick lines represent spectral segments from observations with FAST, SpeX, AKARI, and IRS. The black line is the stitched spectrum. The red line is the best fitting disk+dust+stellar model for the stitched spectrum, while the blue, green, and yellow lines represent the individual disk, dust, and host galaxy components.

where fdisk(λ), fdust(λ), and fstar(λ) are the templates and e −AVτ (λ) is the extinction correction for the disk template. The coefficients a, b, and AV must be the same for all four spectral segments, but coefficient c may take different values in each segment to compensate for the different amounts of host galaxy light in each aperture. We also apply a scaling factor between 0.7 and 1.3 to the fluxes in the FAST, AKARI and IRS segments to correct for any potential biases in the absolute flux calibration of up to ∼30% relative to the SpeX spectrum. The values of these scaling factors are treated as free parameters in the fitting algorithm and they are computed independently for each source.

We use the coefficients in the best fitting model to obtain a stitched spectrum that merges the FAST, SpeX, AKARI, and IRS segments. For this we first multiply each segment (except the SpeX one, which is taken as reference) by its corresponding scaling factor, and then subtract the excess stellar component given by:

$$
\Delta F\_{star}^{i}(\lambda) = (c^{i} - c^{\text{SpeX}}) f\_{star}(\lambda) \tag{2}
$$

where the superscript i indicates the spectral segment. The resulting stitched spectrum corresponds to our prediction of the spectrum that would be observed if all the segments had the same PSF, were extracted in the same aperture, and had perfectly consistent absolute calibrations.

We evaluate the quality of the fits using the normalized root mean square error (RMSE), defined as:

$$RMSE = \sqrt{\frac{1}{N} \sum\_{i}^{N} \frac{(f\_i - F(\lambda\_i))^2}{f\_i^2}} \tag{3}$$

where fi and F(λi) are the flux densities at wavelength λi in the stitched spectrum and the model, respectively. This statistic represents the typical relative residual between the stitched spectrum and the model, and is more informative than χ 2 in situations where residuals are not dominated by noise but differences between the model and the intrinsic spectrum of the source (Hernán Caballero, 2012; Hernán-Caballero et al., 2015).

**Figure 4** shows the original and stitched spectra for the 13 sources in the sample, as well as the best fitting decomposition into disk, dust, and stellar components. The contribution from the stellar component to the total emission varies from negligible (e.g., Mrk 79, Mrk 509, and Mrk 335) to dominant (e.g., Mrk 590), while the interstellar emission is negligible in all but NGC 7469. There is no obvious correlation between the spectral class (Seyfert types 1.0, 1.2, and 1.5) and model parameters such as AV of the disk or the relative luminosities of the components, albeit the sample size is too small to draw any strong conclusions. The fits are remarkably good for most sources (typical RMSE ∼10%), with significant discrepancies arising only at very short (λ < 0.4µm) and long (λ > 7–8µm) wavelengths.

#### REFERENCES

Glikman, E., Helfand, D. J., and White, R. L. (2006). A near-infrared spectral template for quasars. Astrophys. J. 640, 579–591. doi: 10.1086/500098

This suggests that the optical and NIR spectrum of the disk and dust emissions are relatively uniform among type 1 AGN regardless of luminosity. The discrepancies at short wavelength may be caused by uncertainty in the extinction affecting the disk component due to lack of restframe-UV observations, as well as increasing discrepancy between the assumed template for the stellar population and the actual spectrum of the host at short wavelengths. Discrepancies at λ > 7–8µm are caused by sourceto-source variation in the strength of the mid-infrared aromatic features relative to the stellar continuum, and in some cases like Mrk 590, NGC 4151, and PG 0844+349, an unusually strong silicate emission feature at ∼10µm. Hatziminaoglou et al. (2015) showed that accurate modeling of the mid-infrared spectrum of Seyfert 1 s requires a different approach, splitting the stellar and ISM emissions into separate spectral components and using multiple dust templates with different spectral indices and silicate strengths.

#### AUTHOR CONTRIBUTIONS

AH selected the sample, performed most of the data analysis, and wrote the manuscript. EH compiled the broadband observations, contributed to the data analysis and corrected the manuscript. AA and SM contributed to the discussion of the analysis and results and provided further corrections to the manuscript.

#### FUNDING

This work was funded through the Spanish Ministry of Economy and Competitiveness (MINECO) grants AYA2015-70815-ERC, AYA2015-63650-P, and AYA2015-64346-C2-1-P.

#### ACKNOWLEDGMENTS

AH acknowledges support from the ESO Scientific Visitor Programme. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, Caltech under NASA contract 1407. The Cornell Atlas of Spitzer/IRS Sources (CASSIS) is a product of the Infrared Science Center at Cornell University, supported by NASA and JPL. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Glikman, E., Urrutia, T., Lacy, M., Djorgovski, S. G., Mahabal, A., Myers, A. D., et al. (2012). FIRST-2MASS red quasars: transitional objects emerging from the dust. Astrophys. J. 757:51. doi: 10.1088/0004-637X/75 7/1/51

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2017 Hernán-Caballero, Hatziminaoglou, Alonso-Herrero and Mateos. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# NGC 1275: An Outlier of the Black Hole-Host Scaling Relations

Eleonora Sani 1 \*, Federica Ricci 1,2, Fabio La Franca2 , Stefano Bianchi 2 , Angela Bongiorno3 , Marcella Brusa4,5, Alessandro Marconi 6,7, Francesca Onori 8 , Francesco Shankar 9 and Cristian Vignali 4,5

1 European Southern Observatory, Santiago de Chile, Chile, 2 Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Rome, Italy, 3 Istituto Nazionale di Astrofisica Osservatorio Astronomico di Roma, Rome, Italy, 4 Dipartimento di Fisica e Astronomia, Università di Bologna, Bologna, Italy, 5 Istituto Nazionale di Astrofisica Osservatorio Astronomico di Bologna, Bologna, Italy, 6 Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Firenze, Italy, 7 Istituto Nazionale di Astrofisica Osservatorio Astrofisico di Arcetri, Firenze, Italy, 8 SRON Netherlands Institute for Space Research, Utrecht, Netherlands, 9 Department of Physics and Astronomy, University of Southampton, Southampton, United Kingdom

#### Edited by:

Paola Marziani, Osservatorio Astronomico di Padova (INAF), Italy

#### Reviewed by:

Brian Punsly, International Center for Relativistic Astrophysics, Italy Edi Bon, Astronomical Observatory, Serbia

> \*Correspondence: Eleonora Sani<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Milky Way and Galaxies, a section of the journal Frontiers in Astronomy and Space Sciences

Received: 15 November 2017 Accepted: 11 January 2018 Published: 06 February 2018

#### Citation:

Sani E, Ricci F, La Franca F, Bianchi S, Bongiorno A, Brusa M, Marconi A, Onori F, Shankar F and Vignali C (2018) NGC 1275: An Outlier of the Black Hole-Host Scaling Relations. Front. Astron. Space Sci. 5:2. doi: 10.3389/fspas.2018.00002 The active galaxy NGC 1275 lies at the center of the Perseus cluster of galaxies, being an archetypal BH-galaxy system that is supposed to fit well with the MBH-host scaling relations obtained for quiescent galaxies. Since it harbors an obscured AGN, only recently our group has been able to estimate its black hole mass. Here our aim is to pinpoint NGC 1275 on the less dispersed scaling relations, namely the MBH-σ⋆ and MBH − Lbul planes. Starting from our previous work (Ricci et al., 2017a), we estimate that NGC 1275 falls well outside the intrinsic dispersion of the MBH-σ⋆ plane being 1.2 dex (in black hole mass) displaced with respect to the scaling relations. We then perform a 2D morphological decomposition analysis on Spitzer/IRAC images at 3.6 µm and find that, beyond the bright compact nucleus that dominates the central emission, NGC 1275 follows a de Vaucouleurs profile with no sign of significant star formation nor clear merger remnants. Nonetheless, its displacement on the MBH − L3.6,bul plane with respect to the scaling relation is as high as observed in the MBH-σ⋆. We explore various scenarios to interpret such behaviors, of which the most realistic one is the evolutionary pattern followed by NGC 1275 to approach the scaling relation. We indeed speculate that NGC 1275 might be a specimen for those galaxies in which the black holes adjusted to its host.

 OVERVIEW

Although active galactic nuclei (AGN) are divided in many flavors, the Unified Model (Antonucci, 1993) explains these observational properties with a line-of-sight-dependent scenario, in which a dusty torus makes the emission anisotropic. Nowadays however there is growing observational evidence that type 1 AGN (AGN1) and type 2 (AGN2) are actually characterized by intrinsically different physical properties [e.g., different luminosity functions (La Franca et al., 2005; Ueda et al., 2015); accretion rates (Winter et al., 2010); intrinsic X-ray luminosity and black hole (BH) masses (Tueller et al., 2008)]. Thanks to our new virial relation based on unbiased physical quantities, i.e., hard X-ray luminosity and Paβ emission line FWHM, we have been able to measure, for the first time, with virial methods the supermassive black hole mass (MBH) of AGN2 (La Franca et al., 2015, 2016; Onori et al., 2017b), whose values have been up today estimated using scaling relations.

These relations are calibrated on AGN1 and are unlikely to hold also for all AGN2 (Graham, 2008). With direct virial masses for AGN2, we can investigate the relation between the MBH and the bulge properties, thus putting a missing piece to the AGN/galaxy coevolution puzzle. By selecting unbiased samples of type 1 and type 2 AGN, we have found (Onori et al., 2017b) that in the luminosity range 42.5 < log(LX/erg s−1 ) < 44.5, where the two distributions of main AGN types overlap, AGN2 show, on average, significantly smaller Paβ and HeI FWHM than AGN1 (1,970 km/s instead of 3,400 km/s). As expected from the analysis of the FWHM distributions, it results that in this luminosity range, the average MBH of the AGN2 sample [log(MBH/M⊙) = 7.08±0.10] is ∼0.5 dex smaller than measured in the AGN1 sample [log(MBH/M⊙) = 7.61±0.01]. In Ricci et al. (2017a) we have investigated the MBH-σ⋆ relation and seen how AGN2 harbor less massive BHs than AGN1 at a given velocity dispersion, with BH masses of AGN2 ∼ 0.9 dex smaller than AGN1 at σ⋆ ∼ 185 km/s. Equivalently, AGN2 host galaxies have stellar velocity dispersions ∼ 0.2 dex higher than AGN1 hosts at MBH ∼ 107 M⊙. Moreover, such feature is not related to the morphological type but could rather be intrinsic.

To further test this scenario, and to verify whether such a discrepancy holds also for other scaling relations still related to fundamental physical quantities, we are going to investigate the MBH vs. bulge luminosity relation and we present here the case study of NGC 1275.

Throughout this paper we assume the standard cosmology with H0 = 70 km/s/Mpc, M = 0.3, 3 = 0.7.

#### 2. NGC 1275: PROPERTIES, AND BH MASS ESTIMATE

NGC 1275 is the brightest and most massive galaxy in the Perseus Cluster (Abell 426), it is a mildly star-forming early-type cD galaxy with a stellar mass of M⋆ = 2.43×1011 M⊙(Mathews et al., 2006), i.e., the archetypal BH-galaxy system that is supposed to fit well with the MBH-host scaling relations obtained for quiescent galaxies. By adopting such stellar mass and the MBH-M⋆ relation of Sani et al. (2011) (see their Equation 8), one would expect a BH mass of 2.9 × 108 M⊙. Here we want to verify whether such estimate is consistent with direct measurements and investigate the locus of NGC 1275 over the MBH-σ⋆ and MBH-L3.6,bul scaling relations. Since it also harbor a partially obscured/faint AGN, it represents an excellent laboratory to examine the intrinsic differences between AGN1, AGN2, and quiescent BH.

A direct MBH measurement by means of molecular gas kinematics is discussed by Wilman et al. (2005) and more recently by Scharwächter et al. (2013), the latter having the advance of a spatial resolution twice higher than the former. According to Scharwächter et al. (2013) then, the inner (R∼ 50 pc) molecular H2 gas kinematics is consistent with a circumnuclear disk and the authors estimate a black hole mass of 8 × 108 M⊙, which would be slightly overmassive of a factor of 2 with respect to the MBH-M⋆ plane taking into account the rms scatter of the MBH-M⋆ in Kormendy and Ho (2013). Moreover, such estimate is highly uncertain and considered by Scharwächter et al. (2013) as an upper limit for MBH in NGC 1275. Several sources of uncertainties can be identified and are also partly discussed in Scharwächter et al. (2013). (I) The spatial resolution should be such to allow a resolution at least 2.5 times the sphere of influence of the BH (Hicks et al., 2010), while that on IFU data for NGC 1275 can barely relove it. Even though improved by adaptive optics corrections, the resolution of IFU data in Scharwächter et al. (2013) does not allow to resolve well enough the BH sphere of influence. (II) The molecular gas mass in the inner region is non negligible and, just considering it, the BH mass is halved. (III) Since the disk inclination i highly degenerate with MBH, it cannot be a free paramer of the kinematical modeling. Unfortunately it can be measured in very few cases: when the disk itself is spatially resolved, e.g., with water maser observations, or when very high S/N IFU data permit to take into account the residual projected velocity on cos i (see Appendix B in Marconi et al., 2003). This is not the case for Wilman et al. (2005) and Scharwächter et al. (2013) and they need to assume the inclination of the circumnuclear disk, whose rotational axis is supposed to be oriented as the axis of the radio jet. This is a strong assumption, as radio jets may not necessarily be aligned perpendicular to the disk. Pastorini et al. (2007) showed how the broad range of allowed inclinations leads to MBH upper limits rather than reliable measurements. Finally, (IV) the H2 disk kinematics can be significantly affected by bulk motions and non-gravitational forces like gas streams, which are likely to be falling into the core of NGC 1275 (Scharwächter et al., 2013).

As a valid alternative to direct dynamical measurements, we can use single epoch (SE) scaling relations to estimate MBH in AGN, a method that is widely accepted and not expensive in terms of telescope time (Peterson, 2008; Shen, 2013). SE relations are most commonly calibrated by means of either the continuum or broad emission line luminosity and the FWHM of optical emission line (for a review of these calibrations see Shen, 2013). Recently Koss et al. (2017) gave estimates based on broad Hβ FWHM and 5,100 Å continuum luminosity, and on broad Hα FWHM and integrated luminosity, the latter is considered by the authors as more reliable and leads to MBH ∼ 1.4 × 107M⊙. While such optical SE prescriptions are ideal for type 1 AGN, they can be anyhow problematic for a galaxy such as NGC 1275 which is optically classified as Seyfert 1.5 by Ho et al. (1997), and dimmed optical features produce a high uncertainty in the BH mass estimate. For intermediate type 1 Seyfert, the continuum luminosity and the Hβ complex can be polluted by the FeII emission which is difficult to disentangle, and attenuated by gas extinction; extinction also dims significantly the Hβ emission and partially the Hα so that the second estimate is expected to give a higher mass than the first one and a more reliable value. We note that absorption is consistently detected also in X-rays, having NGC 1275 a gas column density of log NH ∼ 21.2 (Tueller et al., 2008).

Rather than on the Balmer serie emission lines, we can resort to infrared features to pierce the absorbing gas and have a direct view of the broad line region. Combined with hard X-ray luminosity, IR features have been used by Ricci et al. (2017b) to

calibrate new and accurate single epoch viral relations that can be used for obscured or faint AGN.

With a hard X-ray luminosity of L14−195 = 5.13 × 1043 erg/s and a broad Paβ with FWHM=2824 km/s (Onori et al., 2017a), we recently estimated a BH mass of (2.9 ± 0.4) × 107 M⊙ for NGC 1275 (Onori et al., 2017b).

Inclination can be a factor of uncertainty also for MBH estimates based on SE scaling relations. The relations depend indeed on the viar factor factor f that account for our ignorance of the morphology, geometry, and kinematics of the BLR. There are evidences for example that more inclined AGN have a larger FWHM of the broad Hbeta (Bisogni et al., 2017), while the f factor decreases with inclination (Risaliti et al., 2011; Pancoast et al., 2014). Considered together these results point out a possible disc-like structure of the BLR. Anyhow, the only way to match the single-epoch based MBH with molecular gas kinematic measurements is to increase f of about an order of magnitude, which can happen only under extreme conditions like an Eddington ratio λEdd < 0.01 and inclination i < 20 deg (see plot 19 and 20 in Pancoast et al., 2014).

Our measure is consistent with Koss et al. (2017) results. Since the two methods are completely independent, we consider SE estimates reliable, while direct kinematical modeling of molecular gas can provide only upper limits of MBH in NGC 1275. The following analysis is therefore based on our value of MBH ∼ 3 × 107 M⊙. Such low value, an order of magnitude lower than what expected for quiescent galaxies and AGN1, is intriguing and deserve further discussion.

#### 3. NGC 1275: AN OUTLIER OF MBH-σ⋆ PLANE

In Ricci et al. (2017a) we have located type 2 AGN having virial BH mass estimates on the MBH-σ⋆ scaling relation for unobscured and quiescent galaxies.

For a median stellar velocity of σ⋆ ∼ 185 km/s this analysis shows that AGN2 harbor black holes with a mass ∼ 0.9 dex smaller than in AGN1 or, equivalently, that host galaxies for type 2 AGN have a hotter kinematics than for type 1, being σ⋆ 0.2 dex higher at MBH ∼ 107 M⊙. Such result does not depend on the host galaxy morphology (i.e., early vs. late type).