Berton & Järvelä

*M. Berton & E. Järvelä
;

# Peaked sources and narrow-line Seyfert 1s: a love story

Marco Berton  Emilia Järvelä  European Southern Observatory (ESO), Alonso de
Córdova 3107, Casilla 19, Santiago 19001, Chile  Finnish Centre for Astronomy
with ESO (FINCA), University of Turku, Vesilinnantie 5, FI-20014 University of
Turku, Finland  Aalto University Metsähovi Radio Observatory, Metsähovintie
114, FI-02540 Kylmälä, Finland  European Space Agency (ESA), European Space
Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la
Cañada, Madrid, Spain<EMAIL_ADDRESS><EMAIL_ADDRESS>
(26 April 2016; 6 June 2016; 6 June 2016)

###### Abstract

The first similarities between peaked sources (PS) and narrow-line Seyfert 1
(NLS1) galaxies were noticed already twenty years ago. Nowadays, it is known
that several sources can share both classifications, and that part of the
parent population of $\gamma$-ray emitting NLS1s could be hiding among PS. In
this brief review, we describe how and why this orientation-based unification
was developed. We also show how the recent discovery of absorbed radio jets in
NLS1s, basically invisible at frequencies below 10 GHz, could impact our
knowledge of PS and, in particular, render the widely used radio-loudness
parameter obsolete.

###### keywords:

Active galactic nuclei; peaked sources; narrow-line Seyfert 1; unified models

††articletype: Article Type

##  1 Introduction

Peaked sources (PS) are a class of powerful ($logP_{1.4\;\rm{GHz}}>25$ W Hz-1)
radio sources characterized by a small linear size ($<$15 kpc) of their
relativistic jets and by a peaked radio spectrum. The PS population is a mixed
bag (O’Dea Saikia, 2021). Some may be variable radio sources with a
temporarily inverted spectrum. Others may be transient sources in which the
spectral shape remains the same over time. In this case, intermittent activity
of the nucleus may turn the jet on and maintain it for 103-104 yrs, and then
turn it off and remain quiescent for a longer interval (104-106 yrs, Czerny .,
2009). Another option is the young age scenario, in which PS represent a
population of young sources that are still developing and will eventually grow
into classical radio galaxies (Fanti ., 1995). Both of these scenarios have in
common the young kinematical age of the relativistic jets.

However, PS are not the only class of sources which may have kinematically
young jets. Classified not based on their radio spectra as PS, but instead on
their optical spectra, narrow-line Seyfert 1 (NLS1) galaxies have by
definition a full-width at half maximum of the H$\beta$ emission line lower
than 2000 km s-1 (Osterbrock Pogge, 1985). Despite such narrow permitted
lines, they are not obscured as Type 2 active galactic nuclei (AGN), but they
are instead Type 1 AGN, in which the broad-line region (BLR) and the central
engine are directly visible. Although alternative hypotheses exist (Decarli .,
2008), the narrowness of permitted lines is typically attributed to low
rotational velocity around a black hole with mass (106-108 M⊙, Peterson
Wandel, 1999) lower than that of other AGN. Since the bolometric luminosity of
NLS1s is comparable to that of more massive AGN, they must be accreting close
to the Eddington limit (e.g., see Komossa, 2018; Gallo, 2018; Lister, 2018;
Foschini, 2020, for reviews on their properties).

These properties led to an interesting hypothesis. Let us consider two AGN,
located at the same redshift, with the same physical structure and the same
life cycle. If one of them has a lower black hole mass it is reasonable to
assume that its black hole has spent less time accreting efficiently in an
active phase. In this sense, the low-mass AGN can be considered younger than
the high-mass one. This could be the case for NLS1s, as they may be the
progenitors of AGN with higher black hole masses (Mathur, 2000), and more
recent statistical analysis seem to confirm this scenario (Fraix-Burnet .,
2017; Järvelä ., 2017).

Some NLS1s harbor powerful relativistic jets that, when closely aligned with
the line of sight (i.e., beamed), can be sources of $\gamma$-ray emission
(Abdo ., 2009). Such jetted NLS1s aligned with our line of sight seem to share
most of the properties of their non-jetted counterparts, such as black hole
mass and Eddington ratio (Berton ., 2015), and for this reason they could be
the progenitors of other $\gamma$-ray sources like flat-spectrum radio quasars
(FSRQs, Foschini ., 2015). However, it is known that, when observed at large
angles, FSRQs appear as high-excitation radio galaxies (HERGs, Urry Padovani,
1995; Giommi ., 2012). We mentioned before that some PS sources could be young
radio galaxies. Therefore, some of these genuinely young PS may be the
misaligned counterpart (i.e., the parent population) of $\gamma$-ray NLS1s
(Berton ., 2017).

##  2 A new unified model?

The similarity between some PS and jetted NLS1s was already noticed twenty
years ago, when the powerful radio source PKS 2004-447 was identified as a PS
with an NLS1 optical spectrum (Oshlack ., 2001). After that, a number of
sources sharing both classifications have been found (Komossa ., 2006; Yuan .,
2008; Gu ., 2015; Liao Gu, 2020; Yao Komossa, 2021). Based on this, Berton .
(2016) proposed that a subclass of PS called low-luminosity compact (LLC)
sources (Kunert-Bajraszewska ., 2010) could be good candidates as part of the
parent population of $\gamma$-ray NLS1s. Some of these LLCs can be optically
classified as Type 2 HERGs (Kunert-Bajraszewska Labiano, 2010), and their
black hole mass and Eddington ratio distributions overlap with those of beamed
NLS1s. Berton . (2016) also studied their radio luminosity function (LF). This
technique allows to add a model of relativistic beaming to the LF of an
unbeamed population, which can be then compared to the observed LF of the
beamed population (Padovani Urry, 1992). This analysis showed that LLC can
belong to beamed NLS1s’ parent population if the diffuse radio emission of the
latter is negligible with respect to their non-thermal core emission. As later
confirmed by Berton . (2018), this is exactly what is observed among beamed
NLS1s that, unlike FSRQs (Antonucci Barvainis, 1988), typically lack diffuse
emission.

A potential issue with this unification is that, while jetted NLS1s tend to be
harbored in spiral galaxies (Berton ., 2019; Järvelä ., 2018; Kotilainen .,
2016; Olguín-Iglesias ., 2020; Hamilton ., 2021), PS are usually hosted by
ellipticals (O’Dea Saikia, 2021). However, in general, HERG-like PS tend to
have bluer colors than LERG, possibly pointing toward a more gas-rich host
(Heckman Best, 2014). Furthermore, to the best of our knowledge no dedicated
study has been carried out for LLC/HERGs. Since their central engine
properties are similar to those of jetted NLS1s, it would not come as a
surprise to find that they also are hosted by spirals. This point, anyway,
clearly shows that jetted NLS1s cannot be unified with the whole population of
PS. Only a very specific subset of them can be part of the parent population
of jetted NLS1s, very likely those with a relatively low-mass black hole and a
high Eddington ratio.

As previously mentioned, there is no doubt that NLS1s and some PS do share
some properties. The two classifications are not mutually exclusive, since
they are based on different frequencies. Several $\gamma$-NLS1s show peaks
above 10 GHz, similarly to what is observed in some PS (Lähteenmäki ., 2017).
Furthermore, radio observations carried out with very long baseline
interferometry (e.g., Schulz ., 2016; Caccianiga ., 2017) showed that some
jetted NLS1s respect the criteria to be classified, in radio, as PS. Two of
these jetted NLS1s with PS classification, PKS 2004-447 and 3C 286 (e.g., Yao
Komossa, 2021; Berton ., 2021), are also $\gamma$-ray sources included in the
Fermi 4FGL. Since this catalog only includes six PS (Abdollahi ., 2020), and
two of them are NLS1s, it is clear that sources with both classifications are
potentially very interesting candidates as high-energy emitters.

##  3 The riddle of invisible jets

The most interesting discovery of the last decade in the NLS1 field (Foschini,
2020) is the detection at 37 GHz of a few radio-quiet or even radio-silent
(i.e. never detected in any radio survey) NLS1s with the Metsähovi Radio
Telescope (Lähteenmäki ., 2018). Seven of these objects were unexpectedly
found flaring multiple times at Jy-level with relatively short timescales. The
only reasonable mechanism to produce this kind of emission is by means of a
relativistic jet, as seen in blazars. Follow-up observations with the VLA
revealed something even more unexpected. These NLS1s all have very faint
emission at lower radio frequencies. Between 1.4 and 10 GHz, their flux
densities are of the order of $\sim$mJy or, more often, $\mu$Jy (Berton .,
2020). The beam sizes of VLA and Metsähovi are very different. This, however,
cannot account for such a variable emission, as it would be located far from
the nucleus or even outside of the host galaxy. Furthermore, new observations
are revealing that radio flares correspond to a brightening of the nucleus at
other frequencies (Romano et al., in prep). This indisputably proves that the
flares are produced by the NLS1 nuclei. What can cause such a peculiar
behavior?

The spectrum of these objects is dominated by a power law with spectral index
$\alpha_{\nu}\sim-0.7$ ($S_{\nu}\propto\nu^{\alpha}$) below 10 GHz. Above this
frequency, instead, the spectrum is rising with a spectral index up to
$\alpha_{\nu}=6.8$. The emission at high frequency was measured only during a
flaring state of the jet, therefore this number is definitely an upper limit.
However, it is evident that the spectrum cannot have the same spectral index
measured at low frequencies up to $\sim$37 GHz, or these flares would lead to
an unrealistic increase in the flux density of $>$4500 times.

An inverted radio spectrum is nothing new among PS, as it is known that small
jets are peaking at relatively high frequencies. But in our case, the peak
would be around 40 GHz, which would be unprecedented, and also short lived,
because it would move to much lower frequencies in a few years. A more likely
explanation for the inverted spectrum we observe is that the origin of the
radio emission in the two frequency domains is different. At low frequency the
jet is not visible, and the radio emission is likely produced by a combination
of nuclear activity and star formation (e.g., supernova remnants), which is
often very strong in NLS1s (Sani ., 2010). The high frequency emission,
instead, originates in a small-scale relativistic jet, possibly closely
aligned with the line of sight as in blazars. We hypothesize that a dense
region of ionized gas is present between the star forming regions and the
small-scale jet. The ionization source may be either young hot stars or a bow
shock produced by the jet itself. The gas is optically thick at low
frequencies because of free-free absorption. For this reason, only the
emission from the star formation, which is occurring farther away, is visible
at low frequencies. At high frequency, instead, the ionized gas is optically
thin, and the jet radio emission can escape freely. The relativistic jet must
have a relatively small scale, otherwise the size and density of the region of
ionized gas would be unrealistically large. This may be in agreement with the
young age scenario for NLS1s. A small-scale jet, indeed, may have formed only
rather recently.

##  4 The death of radio-loudness

The discovery of absorbed relativistic jets needs confirmation by means of
multiwavelength observations. If our hypothesis is correct, however, it could
have severe implications, especially regarding the use of the radio loudness
parameter. Defined by Kellermann . (1989), it is the ratio between optical and
radio flux densities, $R=S_{\rm opt}/S_{\rm radio}$. However, as suggested
already in Padovani (2017) and Järvelä . (2017), AGN should be classified
based on their real physical properties, not based on an artificial parameter
with an arbitrary threshold. The radio loudness parameter had its use when our
knowledge of AGN and different AGN-related phenomena was still limited, and
most studies focused on either very bright and distant quasars, or nearby,
only moderately luminous Seyferts. Due to this bias in interests the so-called
radio loudness dichotomy was formulated (Cirasuolo ., 2003), stating that
there are two intrinsically different populations of AGN: radio-loud and
radio-quiet sources, and no smooth transition between the two populations. In
this scheme the high radio-loudness was associated with bright AGN with
relativistic jets, and the radio-quiet sources were thought to be mostly non
jetted. During the past decades it has become increasingly clear that the
radio-loudness distribution of AGN is not bimodal (e.g., Järvelä ., 2015), and
this view of AGN is too simplistic.

Already 20 years ago Ho Peng (2001) showed that when carefully measuring the
radio-loudness of the nuclear component in radio-quiet Seyfert galaxies,
majority of them would actually be classified as radio-loud. This highlights
one of the issues with the radio-loudness parameter: the host galaxy. In
nearby sources the host galaxy can considerably contribute to the optical
light measured from the galaxy, and decrease the radio-loudness of the source.
This aspect is not taken into account in the original definition of the radio-
loudness parameter. The host galaxy can also have an opposite effect, as shown
in Caccianiga . (2015). They study a sample of radio-loud NLS1s and conclude
based on their mid-infrared properties that actually some of these sources
might appear radio-loud due to the enhanced star formation processes in the
host galaxy.

The radio-loudness parameter is too ambiguous to be reliable, or even useful.
In addition to the issues mentioned above, its value also depends on, for
example, the optical and radio variability of AGN, the instruments used on
either band, and their sensitivity. The final nail to the coffin of radio-
loudness was the discovery of most likely relativistic jets in sources that
were previously classified as radio-quiet or radio-silent. For now these
sources are a curiosity, but we do not know how common these or similar
properties among AGN are. However, this example perfectly highlights the poor
predicting power of the radio-loudness parameter, and it can be easily seen
how using it might lead to misleading or even wrong results, especially among
sources, such as NLS1s, where different components can co-exist. Using such a
parameter to, for example, divide samples in statistical studies would
obviously skew the results, and lessen their reliability and significance.
These problems can be avoided by properly classifying the sources based on
their physical properties, instead of a vague parameter, repeatedly proven to
be misleading. The radio-loudness parameter truly seems to be a relic of the
past, and we thus kindly suggest that we finally lay it to rest.

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## Authors’ Biography

Marco Berton is an ESO fellow, previously working at the Finnish Centre for
Astronomy with ESO. He obtained his PhD from University of Padova in 2016. His
research interest are active galaxies, and particularly NLS1s. Emilia Järvelä
is an ESA research fellow, previously employed at University of California,
Santa Barbara. She obtained her PhD from Aalto University Metsähovi Radio
Observatory in 2018. Her research interests encompass the evolution of active
galaxies, NLS1s, and other young AGN.