Chilingarian et al. Dwarf early-type galaxies in the Coma cluster

17 2023 10.1017/xxxxx

Proceedings of IAU Symposium 379

# Dwarf early-type galaxies in the Coma cluster: internal dynamics, stellar
populations

Igor V. Chilingarian1,2 Kirill A. Grishin3,2 Anton V. Afanasiev4,2 Anton
Mironov5 Daniel Fabricant1 Sean Moran1 Nelson Caldwell1 Ivan Yu. Katkov6,2
Irina Ershova5 1Center for Astrophysics – Harvard and Smithsonian, 60 Garden
St. Cambridge, MA, 02138 USA
2Sternberg Astronomical Institute, Moscow State University, 13 Universitetsky
prospect, Moscow, Russia
3Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris,
France
4LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195, Meudon, France
5Faculty of Space Research, Moscow State University, 1 Leninskie Gory, bld.
52, Moscow, Russia
6New York University Abu Dhabi, Saadiyat Island, PO Box 129188, Abu Dhabi, UAE

###### Abstract

We present preliminary results from our spectroscopic survey of low-luminosity
early-type galaxies in the Coma cluster conducted with the Binospec
spectrograph at the 6.5 m MMT. From spatially-resolved profiles of internal
kinematics and stellar population properties complemented with high-resolution
images, we placed several low-luminosity dEs on the fundamental plane in the
low-luminosity extension of the available literature data. We also discovered
unusual kpc-sized kinematically-decoupled cores in several dwarf galaxies,
which had been probably formed before these galaxies entered the cluster.

###### keywords:

dwarf elliptical galaxies, dynamics of galaxies, stellar populations

††editors: P. Bonifacio, M.-R. Cioni, F. Hammer, M. Pawlowski, and S. Taibi,
eds.

##  1 Introduction

Dwarf elliptical (dE) and lenticular (dS0) galaxies are low-stellar-mass
($M_{*}\mathrel{\hbox{\hbox to0.0pt{\hbox{\lower
4.0pt\hbox{$\sim$}}\hss}\hbox{$<$}}}5\cdot 10^{9}M_{\odot}$) quiescent stellar
systems commonly found in galaxy clusters and massive groups (Sandage &
Binggeli, 1984). The formation and evolution of dE/dS0s are thought to be
influenced by a combination of internal processes, such as stellar feedback
(Dekel & Silk, 1986) and environmental effects, ram pressure stripping of gas
by the hot intracluster medium (Gunn & Gott, 1972) and tidal interactions with
other cluster members as well as the cluster potential (Moore et al., 1996).
Dwarf early-type galaxies are not as simple as they look. Faint low-contrast
spiral arms and bars were found in several ‘luminous’ dE/dS0s in the Virgo and
Abell 496 clusters (Jerjen et al., 2000; Chilingarian et al., 2008b). The
diversity of rotational support in dE/dS0s (Geha et al., 2002), residual
central star formation (Lisker et al., 2006), and young metal-rich embedded
substructures and kinematically decoupled cores (Chilingarian et al., 2007b,
c; Chilingarian, 2009) strengthened their evolutionary connection to more
massive ‘normal’ galaxies (Boselli et al., 2008). However, which evolutionary
mechanism dominates the dE/dS0 formation is still not understood. In 2017–2022
we carried out a spectroscopic campaign to characterise dE/dS0 populations in
three nearby clusters ($0.023<z<0.045$) and assess their evolutionary paths by
obtaining spatially resolved internal kinematics and stellar population
characteristics of several hundred dE/dS0 galaxies.

##  2 Observations, Data Reduction and Analysis

We observed galaxies in the three low-redshift clusters, Coma$=$Abell 1656
($z=0.023$), Abell 2147 ($z=0.035$), and Abell 168 ($z=0.045$) using the high-
throughput multi-object optical spectrograph Binospec (Fabricant et al., 2019)
operated at the f/5 focus of the 6.5 m MMT at Mt. Hopkins, Arizona. The field-
of-view of 16’$\times$15’ at this redshift range allows us to observe up-to
100 galaxies in each multi-slit mask at the spatial resolution of 0.4–0.7 kpc
(corresponding to the typical 0.8” seeing). Our sample includes $>200$
dEs/dS0s in the Coma cluster (8 masks), 30 in Abell 2147 (1 mask), and 50 in
Abell 168 (2 masks). Using tilted slits, we observed a subsample of $\sim 100$
Coma and Abell 168 dEs/dS0s along their major and minor axes. We integrated
for 2–4 h on each mask and reached a surface brightness $\mu_{B}\sim 25.5$ mag
arcsec-2 at S/N=3 pix-1 in the wavelength range $3700<\lambda<5300$ Å. The
1000 gpm grating provides a spectral resolution $R=4800$. This setup is well
suited for internal kinematic studies and stellar population analysis of faint
absorption-line spectra (Chilingarian & Grishin, 2020; Afanasiev et al.,
2023). In addition to the spectroscopic data, we used high-resolution Hubble
Space Telescope images.

We reduced spectroscopic data using the Binospec data reduction pipeline
(Kansky et al., 2019) optimized for low-surface brightness data. We used the
NBursts full spectum fitting code (Chilingarian et al., 2007a) with $R=10000$
simple stellar population models computed with the pegase.hr code (Le Borgne
et al., 2004) and a dedicated grid of synthetic spectra, which model a process
of a ram-pressure-induced starburst (Grishin et al., 2019) for a subsample of
objects with signs of recent quenching. We analyzed images using galfit (Peng
et al., 2010) by using up-to three light profile components and computed an
equivalent half-light radius from several components.

##  3 Preliminary Results

###  3.1 Coma dEs in $\kappa$-space

Figure 1: Left: examples of Coma cluster dEs with positions of Binospec slits
overplotted. Four galaxies were observed along two axes (green and red slits).
Right: $\kappa$-space FP projection, where the results of our analysis for
Coma cluster dEs are shown by purple stars. KDG 64, a large dwarf spheroidal
or a small UDG in the M 81 group (Afanasiev et al., 2023) is marked: its
position is clearly distinct from the remaining dEs. The arrow represent
predicted passive evolution of diffuse post-starburst galaxies.

A good proxy for a dynamical mass of a virialized stellar system is its
position in the Fundamental Plane (FP, Djorgovski & Davis, 1987), which
requires the knowledge of $R_{e}$, $\langle\mu_{e}\rangle$ and $\sigma_{*}$.
Here we use a so-called $\kappa$-space FP projection (Bender et al., 1992),
where one of the axes, $\kappa_{3}$, is related to the dynamical mass-to-light
ratio and another one, $\kappa_{1}$ can be expressed as a logarithm of the
stellar mass. In Fig. 1 we present a literature compilation of structural and
dynamical measurements for dwarf and giant early-type galaxies, ultradiffuse
galaxies (UDGs), bulges, and compact stellar systems. New measurements from
Binospec spectra and HST images are shown as purple stars. The purple stars
fall on the extension of the dE locus towards lower stellar masses and lower
velocity dispersions. Interestingly, these objects seem to have lower
dynamical mass-to-light ratios than Virgo cluster dEs of the same stellar
mass. All fall below the $M/L$ rise in the dwarf spheroidal regime, where KDG
64, one of the most massive dSphs in the Local Volume, resides (Afanasiev et
al., 2023). Interestingly, UDGs have higher $M/L$ ratios for the same stellar
mass.

###  3.2 Kinematically decoupled cores

Figure 2: Examples of kinematically decoupled cores found in VCC 1261 in the
Virgo cluster (re-analysis of archival observations from Keck ESI presented in
Geha et al., 2003) and GMP 3034 (Binospec data). Note the difference of
spatial scales (0.08 and 0.5 kpc arcsec-2 for Virgo and Coma respectively).

Another intriguing result of our project is a discovery of several
kinematically decoupled cores (KDCs) in brighter ($M_{V}<-17$ mag) Coma
cluster dEs. In the Coma cluster, to our surprise, we found KDCs which are
much more extended ($>1$ kpc) than those in the Virgo cluster (see Fig. 2).
Because the distance is 6 times greater, we cannot detect KDCs as compact as
those in the Virgo cluster ($200\dots 300$ pc) in ground-based seeing-limited
observations, however, no ‘large’ KDCs have been found in the Virgo dEs to
date (Toloba et al., 2014). Both ‘small’ KDCs in Virgo dEs and ‘large’ KDCs in
Coma dEs are usually distinct in stellar age and metallicity; they are younger
and more metal-rich. KDCs are thought to form via minor mergers, cold gas
accretion, or galaxy flybys at a low relative velocity. While mergers can
happen in clusters, despite a low probability of dwarf-dwarf interactions, the
two latter channels are practically unfeasible due to the dynamically hot
environment. However, a pre-existing KDC in a disk galaxy can survive its
morphological transformation into a dE during infall. The environmental
phenomena in the massive Coma cluster are expected to be more violent than in
Virgo, therefore more massive galaxies with larger pre-existing KDCs can be
efficiently quenched and transformed into ‘oversized’ dEs with large KDCs.

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