11institutetext: 1Faculty of Physics, Ludwig-Maximilians-Universität,
Scheinerstr. 1, 81679, Munich, Germany
11email<EMAIL_ADDRESS>
# Decoding NGC 7252 as a blue elliptical galaxy

Koshy George 11

Elliptical galaxies with blue optical colours and significant star formation
are hypothesised to be major merger remnants of gas-rich spiral galaxies or
normal elliptical galaxies with a sudden burst of star formation. We present
here a scenario in which blue elliptical galaxies identified in shallow
imaging surveys may fail to recover faint features that are indicative of past
merger activity using a nearby major merger remnant. Based on deep optical
imaging data of the post-merger galaxy, NGC 7252, we demonstrate that the
galaxy can appear as an elliptical galaxy if it is observed at higher
redshifts. The main body and the low surface brightness merger features found
at the outskirts of the galaxy are blue in the optical $g-r$ colour map. We
argue that the higher-redshift blue elliptical galaxies discovered in surveys
as shallow as the SDSS or DECaLS may be advanced mergers whose defining tidal
features fall below the detection limits of the surveys. This should be taken
into consideration during the morphological classification of these systems in
future and ongoing surveys.

###### Key Words.:

galaxy evolution, elliptical and lenticular, galaxy interactions, galaxy star
formation

##  1 Introduction

Massive galaxies in the local Universe are morphologically classified as
elliptical (E), S0, and spirals. E/S0 galaxies are generally observed to be
gas poor, without significant ongoing star formation. Spiral galaxies, on the
other hand, tend to be gas rich and are actively forming stars. This is
reflected as distinct bimodal regions in the optical colour-magnitude diagram
and in diagrams of the stellar mass-star formation rate (Baldry et al., 2004;
Brinchmann et al., 2004; Salim et al., 2007; Noeske et al., 2007; Elbaz et
al., 2007; Daddi et al., 2007). The bimodal nature of star formation in
galaxies could be explained by internal or external processes due to which
star-forming galaxies cease to form new stars. This could be associated with a
morphological change through which a star-forming spiral galaxy can become a
non-star-forming E/S0 galaxy. This is possible through major mergers, in which
two equal-mass star-forming spiral galaxies merge to form a massive elliptical
galaxy. In an environmental process, star-forming spiral galaxies can also
fall into galaxy clusters and groups without subsequent supply of gas, which
halts star formation and transforms the morphology into an S0 galaxy. This is
supported by the redshift evolution of stellar mass buildup in red-sequence
(E/S0) galaxies that occurs at the expense of blue cloud (spiral) galaxies
(Bell et al., 2004; Faber et al., 2007; Brown et al., 2007). However, the star
formation rates of a small fraction of blue colour E/S0 galaxies are as
significant as in spiral galaxies (Fukugita et al., 2004; Schawinski et al.,
2009; Kannappan et al., 2009; Huertas-Company et al., 2010; McIntosh et al.,
2014; Mahajan et al., 2018; Moffett et al., 2019; Dhiwar et al., 2022;
Paspaliaris et al., 2023; Lazar et al., 2023). These galaxies are found to be
in low-density regions with blue optical colours, and they occupy the main
sequence of star-forming galaxies. The formation of blue elliptical galaxies
is hypothesised as follows. Two gas-rich, equal-mass spiral galaxies can merge
to form an elliptical galaxy with significant star formation (this is one of
the channels for elliptical galaxy formation). The star formation will cease
when the gas is exhausted, and the galaxy will change to a normal elliptical
galaxy. The other scenario involves a rejuvenation process in which the normal
elliptical galaxy acquires much gas that then collapses to form new stars.

Figure 1: Optical colour-composite image of NGC 7252 made from legacy survey
$g,r,z$ filter bandpass images (blue, green, and red). The image size is 2.6
$\arcmin$ $\times$ 2.6 $\arcmin$ ($\sim$ 50 kpc $\times$ 50 kpc).

We test the hypothesis of a major merger origin for blue elliptical galaxies
using the known major merger remnant NGC 7252. The galaxy main body is a
single-nucleus merger remnant and represents the final stages of the Toomre
sequence of merging, where the merger remnant will eventually deplete the fuel
for star formation and evolve into an elliptical galaxy (Toomre & Toomre,
1972; Toomre, 1977; Schweizer, 1982). The advanced merger between two gas-rich
spiral galaxies results in tidal tails, shells, and ripples around the main
body in optical imaging (Schweizer, 1982; Dupraz et al., 1990; Wang et al.,
1992; Fritze-v. Alvensleben & Gerhard, 1994; Hibbard et al., 1994).
Significant star formation is detected in the main body and at the outskirts
of the galaxy, with indications of a gaseous disk and possible active galactic
nucleus (AGN) feedback at the centre (George et al., 2018a; Weaver et al.,
2018; George et al., 2018b). The merger and the associated starburst in the
galaxy are understood to have started 600–700 Myr ago (Hibbard & Mihos, 1995;
Chien & Barnes, 2010). The surface brightness profile of the galaxy main body
follows a de Vaucouleurs profile (typical of elliptical galaxies), in which
optical spectroscopy reveals post-starburst features (Schweizer, 1982; Hibbard
& Yun, 1999). The galaxy follows the scaling relations of normal elliptical
galaxies, such as the Faber–Jackson and fundamental plane relation (Lake &
Dressler, 1986; Hibbard & Mihos, 1995; Genzel et al., 2001; Rothberg & Joseph,
2006). We used deep optical imaging data of NGC 7252 to investigate whether
the galaxy shows properties similar to that of blue elliptical galaxies. Blue
elliptical galaxies could have formed from a similar equal-mass spiral galaxy
merger, in which the merger features reach beyond the detection limit of the
wide-field optical surveys (The Sloan Digital Sky Survey (SDSS) and The Dark
Energy Camera Legacy Survey (DECaLS)) based on which these galaxies are
originally classified. We explore the optical $g-r$ colour of the low surface
brightness features in the outskirts and compare the values against the main
body of the galaxy. We place the galaxy at various redshifts as imaged at the
surface brightness limit of the legacy survey imaging used in this work by
correcting for angular size distance and applying cosmological surface
brightness dimming. We adopt a flat Universe cosmology with
$H_{\rm{o}}=71\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, $\Omega_{\rm{M}}=0.27$,
$\Omega_{\Lambda}=0.73$ (Komatsu et al., 2011).

##  2 Data and analysis

Figure 2: Surface brightness map of NGC 7252 made from $r$-band imaging data.
The integrated surface brightness is computed from selected regions along the
low surface brightness merger features marked in differently coloured
polygons. The galaxy main body is marked with a white outline.

Figure 3: $g-r$ color map of NGC 7252. The $g-r$ colour is computed from
selected regions along the low surface brightness merger features marked in
differently coloured polygons. The galaxy main body is marked with a white
outline. The $r$-band surface brightness from selected regions is plotted
against the $g-r$ colour. The colour scheme of points is the same as in the
selected regions.

NGC 7252 (RA:22:20:44.7,Dec:24:40:42) is a nearby major merger remnant with a
spectroscopic redshift (z) $=$ 0.0159 (Rothberg & Joseph, 2006). The optical
$g,r,z$-band imaging data of NGC 7252 were taken from Data Release 10 of the
legacy survey DECaLS (Dey et al., 2019). DECaLS uses the Dark Energy Camera,
consisting of 62 2k $\times$ 4k CCDs with a pixel scale of 0.262 arcsec pix-1
and a 3.2 deg2 field of view at the 4 m Blanco telescope of Cerro Tololo
Inter-American Observatory. The observations are conducted in a dynamic
observing mode in which the exposure times and target field selection can be
changed on the fly, depending on the observing conditions to ensure a uniform
depth across the survey. The median full width at half maximum (FWHM) in the
$g,r,z$ band of the delivered image quality is $\sim$ 1.3, 1.2, and 1.1
arcsec. The photometric calibration was made using the Pan-STARRS1 DR1
photometry through the set of colour transformation equations given in Dey et
al. (2019). The $g,r,z$ band coadded images we used in our analysis were
calibrated with pixel values stored in nannomaggies, which can be converted
into magnitudes using the appropriate conversion stored in the header. DECaLS
imaging reaches $\sim$ 2 mag deeper than that of the SDSS and hence can detect
low surface brightness features in the r band down to 28 mag arcsec-2 (the
corresponding limit for SDSS is 25 mag arcsec-2) (Driver et al., 2016; Hood et
al., 2018).

We used the $grz$ imaging data to create a colour-composite image of NGC 7252
by assigning blue ($g$), green ($r$), and red ($z$) colours. Figure 1 shows
the $grz$ colour-composite image of NGC 7252. We note that the pixels from the
central region of the galaxy main body are saturated in $r$-band imaging data.
The region covered by saturated pixels was masked and was not used for further
analysis. Faint tidal features from the recent merger activity are detected
around the galaxy. The morphological features around the galaxy were evaluated
using the $r$-band imaging. The surface brightness map was created from the
$r$-band imaging and is shown in Figure 2 with an inverted greyscale to detect
faint low surface brightness features. We attempted to bring out the faint
features by smoothing the pixel noise through running a Gaussian of
$\sigma$=1. We visually selected different regions by avoiding foreground
stars (and likely stellar clusters) outside the galaxy that we indicated with
colour polygons, and the galaxy main body is marked with a white contour. The
optical $g-r$ color map of the galaxy was created from the flux-calibrated
$g,r$ coadded images ($g-r$ = -2.5 $\times$ log10(fluxg/fluxr)) and is shown
in Figure 3. The $g,r$ surface brightness was computed for the marked regions
of faint features in the outskirts and the galaxy main body. The $r$-band
surface brightness is plotted against the $g-r$ colour of the regions and is
shown in Figure 3. The selected merger features around the galaxy have
integrated blue colours with a median $g-r$ = 0.52. The main body of the
galaxy has an early-type morphology with $g-r$ colour = 0.42. The colour value
should be treated as a lower limit estimate as the central region saturated
pixels in $r$-band imaging are masked and were not used to compute the colour.

We now investigate the appearance of the galaxy at different redshifts in
increments of 0.1 up to z=1. We changed the galaxy size for the changing
angular size distance with redshift and also took the effect of the
cosmological surface brightness dimming ($\mu+10\times log_{10}(1+z)$) on the
surface brightness map ($\mu$) of the galaxy into account. The $r$-band
surface brightness maps for different redshifts between $0>z>1$ are shown in
Figure 4. We limited the surface brightness to 28 mag/arcsec2 at the detection
limit of the legacy survey for every redshift. The surface brightness values
that reach beyond this limit are not shown and will not be detected at that
redshift. This is the expected $r-band$ appearance of NGC 7252 at different
redshifts when observed with a 4m telescope. We note that this is a very
simplified scenario that we put forward here for galaxies with a flat spectral
energy distribution. The observed $r-band$ in reality will be receiving
photons emitted from the rest frame $u-band$ at z $\sim$ 1\.

We show the surface brightness change as a function of redshift due to the
cosmological surface brightness dimming in Figure 5. The dotted black lines
show the change in three different values of $\mu$ from z =0 to 1. Limiting
surface brightness for SDSS and DECaLS surveys are shown by blue and green
horizontal lines. The black point is the measured surface brightness for the
main body of NGC 7252 and the coloured points are the surface brightness of
the faint merger features seen at outskirts as defined in Figure 2.

Figure 4: Surface brightness map of NGC 7252 made from $r$-band imaging data
as it could have appeared in different redshifts. The corresponding 1$\arcsec$
to kiloparsec conversion for each redshift is given inside the plots. A movie
version of the plots is available online. Figure 5: Surface brightness
variation with redshift due to the cosmological dimming (dotted black line).
The surface brightness detection limits from the SDSS and DECaLS sky surveys
are shown as green and blue lines. The integrated surface brightness of the
main body of the galaxy is shown with a black point. The $r$-band surface
brightness from selected regions at the outskirts is shown as coloured points,
as in Figure 2.

##  3 Discussion

The $\Lambda$ cold dark matter paradigm predicts that elliptical galaxies are
formed through a hierarchical merging scenario (De Lucia et al., 2006).
Multiple mergers involving different mass scales are possible, and the gas
content dictates the star formation properties of the merger remnant. NGC 7252
is a nearby major merger remnant understood to have formed from a recent ($<$
700 Myr) equal-mass merger between two gas-rich spiral galaxies that created
the main body around which tidal tails, shells, and ripples are formed, as
shown in the optical colour-composite image in Figure 1. The main body of the
galaxy shows properties typical of elliptical galaxies. The galaxy, however,
resides in the blue cloud of the galaxy colour-magnitude diagram (Weaver et
al., 2018), which is at odds with the normal elliptical galaxies. The stellar
mass for NGC 7252 is computed to be $\sim$ 1010.6 M$\odot$ (Weaver et al.,
2018).

Optical imaging data from wide-field surveys such as SDSS with an integration
time $\sim$ 54sec were used to classify blue E/S0 galaxies based on morphology
(Fukugita et al., 2004; Schawinski et al., 2009). Schawinski et al. (2009)
identified 204 blue E/S0 galaxies using the Galaxy Zoo classification, which
has blue $u-r$ colours that are significantly bluer than the red sequence and
are well within the blue cloud in the optical colour-magnitude diagram
occupied by star-forming galaxies. The redshifts of these galaxies are 0.02
$<z<$ 0.05, and the luminosities are greater than L$\star$. They are found to
be in lower-density environments than red sequence early-type galaxies and
make up $\sim$ 6$\%$ of the low-redshift early-type galaxy population. Based
on an analysis using emission line diagnostic diagrams, 25 $\%$ of these
galaxies are actively star forming, 25 $\%$ host both star formation and an
AGN, 12 $\%$ have an AGN, and 38 $\%$ show no strong emission lines that could
be classified. With star formation rates ranging from 0.5 to 50 M$\odot$/yr,
the star-forming blue E/S0 are found to be hosting intense, spatially extended
star formation. We are interested in understanding the formation of the star-
forming population of blue elliptical galaxies. Blue S0 galaxies can have
different formation scenarios.

We used deeper optical imaging data to investigate whether the star-forming
blue elliptical galaxies share a common origin with NGC 7252. We created a
$g-r$ color map and measured the $g-r$ colours of selected merger features
around the galaxy. We found them to be of blue colours with a median $g-r$ =
0.52. The very central region of the galaxy has blue colors as well (g-r
$\simeq$ 0.42) and is coincident with a star-forming disc revealed by HST
(Whitmore et al., 1993) and UVIT far and near ultraviolet imaging (George et
al., 2018b). The main body of the galaxy has an elliptical morphology, which
means that if it were detected without the tidal features, it would likely be
classified as a blue elliptical galaxy. We note that significant neutral
hydrogen (4.5 $\times$ 109 M$\odot$) and molecular hydrogen (3.5 $\times$ 109
M$\odot$) gas is detected, which indicates that a gas-rich wet merger scenario
is responsible for the formation of the merger remnant (Hibbard et al., 1994;
Wang et al., 1992). The blue elliptical galaxies discussed in Schawinski et
al. (2009) have $>$ L$\star$ luminosity and therefore have a similar stellar
mass as NGC 7252.

We explored the appearance of NGC 7252 at higher redshift with the likely
detection of low surface brightness features from the merger. Figure 4 shows
that by redshift 0.7, the galaxy would not have detectable merger features in
shallow surveys. This suggests that a merger remnant like NGC 7252 residing at
z $\sim$ 0.7 would be morphologically classified as a blue elliptical galaxy.
This is further demonstrated with the position of the galaxy main body on the
grid of changing surface brightness for different redshifts between $0>z>1$ in
Figure 5. The surface brightness of the main body of NGC 7252, shown with a
black point, should be observed as an elliptical galaxy with a blue colour up
to z$\sim$1\. The merger features seen at the outskirts shown with colored
points will disappear faster for shallow surveys such as the SDSS by z $\sim$
0.1, but will appear for deeper surveys such as DECaLS till z $\sim$ 0.6. We
note that this plot only applies to sources with a flat spectral energy
distribution, for which bandpass shifting plays a very negligible role.

We present here an idealised scenario for a merger remnant galaxy in the
nearby Universe (z=0.0159) as it would appear at redshifts up to z $\sim$ 1\.
The galaxy can be more compact at high redshifts in wide-field imaging, which
facilitates a location in the morphological classification as a blue
elliptical. We did not consider the size reduction for elliptical galaxies
that we observe at higher redshifts (Trujillo et al., 2007). This effect is
more prominent for massive elliptical galaxies, and minor mergers are likely
responsible for galaxies that systematically increase their size at low
redshifts (Trujillo et al., 2011). We note that the major merger rate
decreases towards lower redshifts, which can explain the low fraction of blue
elliptical galaxies (Lotz et al., 2011).

Features around merger remnant galaxies can disappear with time since the
merger (Ji et al., 2014). The NGC 7252 merger features are seen very clearly,
implying a recent ($<$ 700 Myr) merger event. We note that almost all blue
elliptical galaxies from Schawinski et al. (2009), although at varying levels,
host features that are indicative of recent mergers revealed from a structural
analysis and deep-imaging data (George & Zingade, 2015; George, 2017, 2023).
The blue elliptical galaxies reported in the shallow SDSS imaging may be seen
for longer time since the merger compared to the case of NGC 7252. Ongoing and
future wide-field deep optical surveys (DES, DECaLS, Euclid, and LSST) will
likely detect more blue elliptical galaxies at higher redshifts, and will need
adequate surface brightness sensitivity to reveal features that are indicative
of recent merger activity.

##  4 Summary

Blue elliptical galaxies are interesting systems for understanding galaxy
formation and evolution. We demonstrated based on deep-imaging data that the
main body of the post-merger galaxy NGC 7252 can appear as a blue elliptical
galaxy if it is observed at higher redshifts. With the evolution of the
stellar population in the main body and the galaxy outskirts, the galaxy will
most likely evolve into a normal elliptical galaxy with a red colour, hosting
evolved stars. We argue that the blue elliptical galaxies found from shallow
imaging surveys may be post-merger systems, with the merger features going
beyond the detection limit of the surveys.

###### Acknowledgements.

The Legacy Surveys consist of three individual and complementary projects: the
Dark Energy Camera Legacy Survey (DECaLS; Proposal ID 2014B-0404; PIs: David
Schlegel and Arjun Dey), the Beijing-Arizona Sky Survey (BASS; NOAO Prop. ID
2015A-0801; PIs: Zhou Xu and Xiaohui Fan), and the Mayall z-band Legacy Survey
(MzLS; Prop. ID 2016A-0453; PI: Arjun Dey). DECaLS, BASS and MzLS together
include data obtained, respectively, at the Blanco telescope, Cerro Tololo
Inter-American Observatory, NSF’s NOIRLab; the Bok telescope, Steward
Observatory, University of Arizona; and the Mayall telescope, Kitt Peak
National Observatory, NOIRLab. Pipeline processing and analyses of the data
were supported by NOIRLab and the Lawrence Berkeley National Laboratory
(LBNL). The Legacy Surveys project is honored to be permitted to conduct
astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular
significance to the Tohono O’odham Nation. NOIRLab is operated by the
Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation. LBNL is managed by
the Regents of the University of California under contract to the U.S.
Department of Energy. This project used data obtained with the Dark Energy
Camera (DECam), which was constructed by the Dark Energy Survey (DES)
collaboration. Funding for the DES Projects has been provided by the U.S.
Department of Energy, the U.S. National Science Foundation, the Ministry of
Science and Education of Spain, the Science and Technology Facilities Council
of the United Kingdom, the Higher Education Funding Council for England, the
National Center for Supercomputing Applications at the University of Illinois
at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the
University of Chicago, Center for Cosmology and Astro-Particle Physics at the
Ohio State University, the Mitchell Institute for Fundamental Physics and
Astronomy at Texas A&M University, Financiadora de Estudos e Projetos,
Fundacao Carlos Chagas Filho de Amparo, Financiadora de Estudos e Projetos,
Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro,
Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and the
Ministerio da Ciencia, Tecnologia e Inovacao, the Deutsche
Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy
Survey. The Collaborating Institutions are Argonne National Laboratory, the
University of California at Santa Cruz, the University of Cambridge, Centro de
Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid, the
University of Chicago, University College London, the DES-Brazil Consortium,
the University of Edinburgh, the Eidgenossische Technische Hochschule (ETH)
Zurich, Fermi National Accelerator Laboratory, the University of Illinois at
Urbana-Champaign, the Institut de Ciencies de l’Espai (IEEC/CSIC), the
Institut de Fisica d’Altes Energies, Lawrence Berkeley National Laboratory,
the Ludwig Maximilians Universitat Munchen and the associated Excellence
Cluster Universe, the University of Michigan, NSF’s NOIRLab, the University of
Nottingham, the Ohio State University, the University of Pennsylvania, the
University of Portsmouth, SLAC National Accelerator Laboratory, Stanford
University, the University of Sussex, and Texas A&M University. BASS is a key
project of the Telescope Access Program (TAP), which has been funded by the
National Astronomical Observatories of China, the Chinese Academy of Sciences
(the Strategic Priority Research Program “The Emergence of Cosmological
Structures” Grant XDB09000000), and the Special Fund for Astronomy from the
Ministry of Finance. The BASS is also supported by the External Cooperation
Program of Chinese Academy of Sciences (Grant 114A11KYSB20160057), and Chinese
National Natural Science Foundation (Grant 12120101003, 11433005). The Legacy
Survey team makes use of data products from the Near-Earth Object Wide-field
Infrared Survey Explorer (NEOWISE), which is a project of the Jet Propulsion
Laboratory/California Institute of Technology. NEOWISE is funded by the
National Aeronautics and Space Administration. The Legacy Surveys imaging of
the DESI footprint is supported by the Director, Office of Science, Office of
High Energy Physics of the U.S. Department of Energy under Contract No. DE-
AC02-05CH1123, by the National Energy Research Scientific Computing Center, a
DOE Office of Science User Facility under the same contract; and by the U.S.
National Science Foundation, Division of Astronomical Sciences under Contract
No. AST-0950945 to NOAO.

## References

  * Baldry et al. (2004) Baldry, I. K., Glazebrook, K., Brinkmann, J., et al. 2004, ApJ, 600, 681. doi:10.1086/380092
  * Bell et al. (2004) Bell, E. F., Wolf, C., Meisenheimer, K., et al. 2004, ApJ, 608, 752. doi:10.1086/420778
  * Brinchmann et al. (2004) Brinchmann, J., Charlot, S., White, S. D. M., et al. 2004, MNRAS, 351, 1151
  * Brown et al. (2007) Brown, M. J. I., Dey, A., Jannuzi, B. T., et al. 2007, ApJ, 654, 858. doi:10.1086/509652
  * Chien & Barnes (2010) Chien, L.-H., & Barnes, J. E. 2010, MNRAS, 407, 43
  * Daddi et al. (2007) Daddi, E., Dickinson, M., Morrison, G., et al. 2007, ApJ, 670, 156.
  * Dey et al. (2019) Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168. doi:10.3847/1538-3881/ab089d
  * De Lucia et al. (2006) De Lucia, G., Springel, V., White, S. D. M., et al. 2006, MNRAS, 366, 499. doi:10.1111/j.1365-2966.2005.09879.x
  * Dhiwar et al. (2022) Dhiwar, S., Saha, K., Dekel, A., et al. 2022, arXiv:2211.08884
  * Driver et al. (2016) Driver, S. P., Wright, A. H., Andrews, S. K., et al. 2016, MNRAS, 455, 3911. doi:10.1093/mnras/stv2505
  * Dupraz et al. (1990) Dupraz, C., Casoli, F., Combes, F., & Kazes, I. 1990, A&A, 228, L5
  * Elbaz et al. (2007) Elbaz, D., Daddi, E., Le Borgne, D., et al. 2007, A&A, 468, 33.
  * Faber et al. (2007) Faber, S. M., Willmer, C. N. A., Wolf, C., et al. 2007, ApJ, 665, 265. doi:10.1086/519294
  * Fritze-v. Alvensleben & Gerhard (1994) Fritze-v. Alvensleben, U., & Gerhard, O. E. 1994, A&A, 285, 751
  * Fukugita et al. (2004) Fukugita, M., Nakamura, O., Turner, E. L., et al. 2004, ApJ, 601, L127. doi:10.1086/382151
  * Genzel et al. (2001) Genzel, R., Tacconi, L. J., Rigopoulou, D., Lutz, D., & Tecza, M. 2001, ApJ, 563, 527
  * George & Zingade (2015) George, K., & Zingade, K. 2015, A&A, 583, A103
  * George (2017) George, K. 2017, A&A, 598, A45
  * George et al. (2018a) George, K., Joseph, P., Côté, P., et al. 2018, A&A, 614, A130. doi:10.1051/0004-6361/201832705
  * George et al. (2018b) George, K., Joseph, P., Mondal, C., et al. 2018, A&A, 613, L9. doi:10.1051/0004-6361/201833232
  * George (2023) George, K. 2023, A&A, submitted
  * Hibbard et al. (1994) Hibbard, J. E., Guhathakurta, P., van Gorkom, J. H., & Schweizer, F. 1994, AJ, 107, 67
  * Hibbard & Mihos (1995) Hibbard, J. E., & Mihos, J. C. 1995, AJ, 110, 140
  * Hibbard & Yun (1999) Hibbard, J. E., & Yun, M. S. 1999, ApJ, 522, L93
  * Hood et al. (2018) Hood, C. E., Kannappan, S. J., Stark, D. V., et al. 2018, ApJ, 857, 144. doi:10.3847/1538-4357/aab719
  * Huertas-Company et al. (2010) Huertas-Company, M., Aguerri, J. A. L., Tresse, L., et al. 2010, A&A, 515, A3. doi:10.1051/0004-6361/200913188
  * Ji et al. (2014) Ji, I., Peirani, S., & Yi, S. K. 2014, A&A, 566, A97. doi:10.1051/0004-6361/201423530
  * Kannappan et al. (2009) Kannappan, S. J., Guie, J. M., & Baker, A. J. 2009, AJ, 138, 579. doi:10.1088/0004-6256/138/2/579
  * Komatsu et al. (2011) Komatsu, E., Smith, K. M., Dunkley, J., et al. 2011, ApJS, 192, 18. doi:10.1088/0067-0049/192/2/18
  * Lake & Dressler (1986) Lake, G., & Dressler, A. 1986, ApJ, 310, 605
  * Lazar et al. (2023) Lazar, I., Kaviraj, S., Martin, G., et al. 2023, MNRAS. doi:10.1093/mnras/stad224
  * Lotz et al. (2011) Lotz, J. M., Jonsson, P., Cox, T. J., et al. 2011, ApJ, 742, 103. doi:10.1088/0004-637X/742/2/103
  * Mahajan et al. (2018) Mahajan, S., Drinkwater, M. J., Driver, S., et al. 2018, MNRAS, 475, 788. doi:10.1093/mnras/stx3202
  * McIntosh et al. (2014) McIntosh, D. H., Wagner, C., Cooper, A., et al. 2014, MNRAS, 442, 533. doi:10.1093/mnras/stu808
  * Moffett et al. (2019) Moffett, A. J., Phillipps, S., Robotham, A. S. G., et al. 2019, MNRAS, 489, 2830. doi:10.1093/mnras/stz2237
  * Noeske et al. (2007) Noeske, K. G., Weiner, B. J., Faber, S. M., et al. 2007, ApJ, 660, L43
  * Paspaliaris et al. (2023) Paspaliaris, E.-D., Xilouris, E. M., Nersesian, A., et al. 2023, A&A, 669, A11. doi:10.1051/0004-6361/202244796
  * Rothberg & Joseph (2006) Rothberg, B., & Joseph, R. D. 2006, AJ, 131, 185
  * Salim et al. (2007) Salim, S., Rich, R. M., Charlot, S., et al. 2007, The Astrophysical Journal Supplement Series, 173, 267.
  * Schawinski et al. (2009) Schawinski, K., Lintott, C., Thomas, D., et al. 2009, MNRAS, 396, 818
  * Schweizer (1982) Schweizer, F. 1982, ApJ, 252, 455. doi:10.1086/159573
  * Toomre & Toomre (1972) Toomre, A. & Toomre, J. 1972, ApJ, 178, 623. doi:10.1086/151823
  * Toomre (1977) Toomre, A. 1977, Evolution of Galaxies and Stellar Populations, 401
  * Trujillo et al. (2007) Trujillo, I., Conselice, C. J., Bundy, K., et al. 2007, MNRAS, 382, 109. doi:10.1111/j.1365-2966.2007.12388.x
  * Trujillo et al. (2011) Trujillo, I., Ferreras, I., & de La Rosa, I. G. 2011, MNRAS, 415, 3903. doi:10.1111/j.1365-2966.2011.19017.x
  * Wang et al. (1992) Wang, Z., Schweizer, F., & Scoville, N. Z. 1992, ApJ, 396, 510
  * Weaver et al. (2018) Weaver, J., Husemann, B., Kuntschner, H., et al. 2018, A&A, 614, A32. doi:10.1051/0004-6361/201732448
  * Whitmore et al. (1993) Whitmore, B. C., Schweizer, F., Leitherer, C., et al. 1993, AJ, 106, 1354. doi:10.1086/116732