MNRAS 497, 1904–1924 (2020) doi:10.1093/mnras/staa2097
Advance Access publication 2020 July 20
On the accretion of a new group of galaxies on to Virgo: I. Internal
kinematics of nine in-falling dEs
Bahar Bidaran,1‹ Anna Pasquali ,1 Thorsten Lisker,1 Lodovico Coccato,2 Jesus Falco´n-Barroso,3,4
Glenn van de Ven ,5 Reynier Peletier ,6 Eric Emsellem,2 Eva K. Grebel,1 Francesco La Barbera ,7
Joachim Janz,8,9 Agnieszka Sybilska,10 Rukmani Vijayaraghavan,11 John Gallagher, III12
and Dimitri A. Gadotti 2
1Astronomisches Rechen-Institut, Zentrum fu¨r Astronomie der Universita¨t Heidelberg, Mo¨nchhofstraße 12-14, D-69120 Heidelberg, Germany
2European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany
3Instituto de Astrofı´sica de Canarias, Calle Vı´a La´ctea s/n, E-38205 La Laguna, Tenerife, Spain
4Departamento de Astrofı´sica, Universidad de La Laguna, E-38200 La Laguna, Tenerife, Spain
5Department of Astrophysics, University of Vienna, Tu¨rkenschanzstrasse 17, A-1180 Vienna, Austria
6Kapteyn Astronomical Institute, University of Groningen, Postbus 800, NL-9700 AV Groningen, the Netherlands
7INAF-Osservatorio Astronomico di Capodimonte, sal. Moiariello 16, I-80131 Napoli, Italy
8Space Science and Astronomy, University of Oulu, PO Box 3000, FI-90014 Oulu, Finland
9Finnish Centre of Astronomy with ESO (FINCA), University of Turku, Va¨isa¨la¨ntie 20, FI-21500 Piikki’o, Finland
10Sybilla Technologies, Torunska 59, PL-85-023 Bydgoszcz, Poland
11Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA
12Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter Street, Madison, WI 53076-1582, USA
Accepted 2020 July 6. Received 2020 July 6; in original form 2020 May 12
ABSTRACT
Galaxy environment has been shown to play an important role in transforming late-type, star-forming galaxies to quiescent
spheroids. This transformation is expected to be more severe for low-mass galaxies (M < 1010 M) in dense galaxy groups and
clusters, mostly due to the influence of their past host haloes (also known as pre-processing) and their present-day environments.
For the first time, in this study, we investigate a sample of nine early-type dwarf galaxies (dEs) that were accreted as a likely
bound group on to the Virgo galaxy cluster about 2–3 Gyr ago. Considering this special condition, these nine dEs may provide
a test bed for distinguishing between the influence of the Virgo galaxy cluster and the effects of the previous host halo on their
current properties. Specifically, we use VLT/MUSE integral-field unit spectra to derive their kinematics and specific angular
momentum (λR) profiles. We observe a spread in the λR profiles of our sample dEs, finding that the λR profiles of half of them
are as high as those of low-mass field galaxies. The remaining dEs exhibit λR profiles as low as those of Virgo dEs that were
likely accreted longer ago. Moreover, we detect nebular emission in one dE with a gas velocity offset suggesting ongoing gas
stripping in Virgo. We suggest that the low-λR dEs in our sample were processed by their previous host halo, prior to their infall
to Virgo, and that the high-λR dEs may be experiencing ram pressure stripping in Virgo.
Key words: galaxies: dwarf – galaxies: evolution – galaxies: interactions – galaxies: kinematics and dynamics – galaxies: struc-
ture.
1 IN T RO D U C T I O N
Of the many different types of galaxies in the Universe, dwarf
elliptical galaxies (dEs) are among the most numerous ones by
number (Sandage, Binggeli & Tammann 1985; Binggeli, Sandage &
Tammann 1988; Trentham & Tully 2002). The probability of
observing dEs in high-density environments, such as groups and
clusters, is higher than in the field (Binggeli et al. 1988; Jerjen &
Tammann 1997; Boselli & Gavazzi 2006; Geha et al. 2012). Studies
of the morphology within one effective radius of these galaxies
 E-mail: bidaran@uni-heidelberg.de
have shown that their structure is not as uniform as previously
thought, since some show different kinds of substructures, for
instance nuclei, spiral arms, and discs (e.g. Lisker, Grebel & Binggeli
2006a). The observed substructures such as disc components in bright
dEs (Binggeli & Cameron 1991; Jerjen, Kalnajs & Binggeli 2000;
Barazza, Binggeli & Jerjen 2002; De Rijcke et al. 2003; Graham &
Guzma´n 2003; Lisker et al. 2006a, 2007), the projected axial ratio
distribution of cluster dEs (Lisker et al. 2007), as well as their
prolonged star formation history (Michielsen et al. 2008; Paudel
et al. 2010; Koleva et al. 2011) and complex internal dynamics
(Geha, Guhathakurta & van der Marel 2003), favour two possible
formation channels: through merging (de Rijcke et al. 2005) or
through transformation of star-forming progenitors into quiescent
C© 2020 The Author(s)
Published by Oxford University Press on behalf of the Royal Astronomical Society
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This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society at https://doi.org/10.1093/mnras/staa2097. © 2020 The Authors.
Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Dwarf kinematics in a group accreted to Virgo 1905
dwarf galaxies due to environmental effects (see e.g. Kormendy
1985; Binggeli et al. 1988; Bender, Burstein & Faber 1992; Moore
et al. 1996; Geha et al. 2010; Toloba et al. 2011; Janz et al. 2012;
Kormendy & Bender 2012; Lisker et al. 2013; Rys´, Falco´n-Barroso &
van de Ven 2013; Boselli & Gavazzi 2014; Penny et al. 2014; Bialas
et al. 2015; Aguerri 2016; Hwang et al. 2018).
In fact, upon accretion on to a cluster or a group (Binggeli,
Tammann & Sandage 1987; Conselice, Gallagher & Wyse 2001),
galaxies undergo transformations as the result of their interactions
with the higher density environment and members of the host halo.
At intermediate cluster-centric distances, ram pressure stripping
can efficiently remove gas and suppress star formation relatively
fast in less massive dwarf progenitors (Gunn & Gott 1972; Lin &
Faber 1983; Kormendy 1985; Boselli & Gavazzi 2006; Chung et al.
2007; Boselli et al. 2014). The gravitational interactions between
the infalling galaxy and the cluster potential well during their orbits
(Smith et al. 2015) as well as close encounters with other cluster
members can trigger morphological and kinematic disturbances (e.g.
destroying the disc substructure, reducing the rotation of the stellar
component, and removing dark matter and stellar mass) within
several orbital periods (Mastropietro et al. 2005; Boselli & Gavazzi
2006; Lisker et al. 2006a; Lisker et al. 2013; Bialas et al. 2015;
Falco´n-Barroso, Lyubenova & van de Ven 2015). Such environmental
mechanisms are more severe for low-mass galaxies, mainly due to
their shallow potential wells. For instance, as shown in the kinematic
studies by Toloba et al. (2011, 2015), the dense core of the Virgo
cluster is mostly populated by dEs that are preferentially pressure-
supported, while most dEs in the cluster outskirts show a higher
degrees of rotation (see also Boselli et al. 2014).
Galaxy rotation can be quantified in terms of specific angular
momentum (λR). λR is a proxy for the projected angular momentum
of a galaxy, which is often described as an indispensable metric
in studies of galaxy evolution (Emsellem et al. 2007; 2011; Jesseit
et al. 2009). Through the evolution of a given galaxy, this parameter
is affected by gas accretion and star formation activity as well as
by the interaction of the galaxy with its surrounding environment
(see Jesseit et al. 2009; Naab et al. 2014; Yozin & Bekki 2016;
Penoyre et al. 2017; Walo-Martı´n et al. 2020). This makes λR a
valuable, but degenerate, metric for a better understanding of galaxy
formation and evolution. Investigations of Rys´ et al. (2014, hereafter
R14) show that dEs in Virgo (as well as two dEs in the field)
tend to show flat λR profiles up to one effective radius. A similar
rather flat profile is also observed for dwarf galaxies in Fornax (see
Scott et al. 2020). As suggested by R14, if host haloes, such as
clusters, are in charge of any transformation in the kinematics of
dE progenitors, then at fixed stellar mass, cluster dEs with different
infall time are expected to exhibit different internal kinematics. In
this picture, dEs with a more recent infall time are expected to show
internal kinematics intermediate between low-mass field galaxies and
cluster dEs accreted at earlier times. In fact, studies of Toloba et al.
(2011) show that in the outer parts of the Virgo cluster and at a fixed
range of luminosity, dEs tend to show similar rotation curves to late-
type galaxies. This is particularly expected in a dynamically young
galaxy cluster with ongoing processes of assembly and a diverse, yet
complete population of galaxies, such as Virgo (Boselli & Gavazzi
2006; Boselli et al. 2018). In contrast, if dEs are already systems
that formed with low and flat λR profiles (Wheeler et al. 2017),
their environment or infall time will not significantly modify their
specific angular momentum. Observations of dEs with low degrees of
rotation in the field by Janz et al. (2017) and Graham et al. (2017) also
challenge the idea that only tidal interactions in clusters can decrease
λR.
Figure 1. Top panel: Spatial distribution of dEs in the Virgo cluster. In both
plots, squares represent the distribution of our sample of dEs colour-coded
based on their λRe (see Section 4.3). The dEs in the Rys´, van de Ven & Falco´n-
Barroso (2014) sample (R14) are denoted with circles and colours that are
scaled accordingly. The same applies to the dEs in the Toloba et al. (2015)
sample (T15) that are denoted with star symbols. The grey circles represent
the distribution of Virgo dEs with −17 ≥ Mr > −18. Positions of Virgo giant
early-type galaxies (from left to right: M60, M87, M49, M86) are marked
with black crosses. Bottom panel: projected phase-space distribution of our
sample of dEs and those of R14. The different labelled zones of the diagram,
taken from Pasquali et al. (2019) and Smith et al. (2019), represent different
average infall times to the host halo (shown here for the Virgo cluster).
A recently accreted group of galaxies in the Virgo cluster was
discovered by Lisker et al. (2018), based on the clustering of nine
dEs with −17 ≥ Mr > −18 in a particular region of the observer’s
phase-space plane. These relatively young dEs (based on their NUV-r
colour) are found at a projected cluster-centric distance of ≈1.5 Mpc
with respect to M87, and move with line-of-sight velocities of
≈700 km s−1 relative to Virgo. Most of them feature substructures,
such as discs and spiral arms (Lisker et al. 2006a,b, 2018). They show
a sparsely defined ellipsoidal distribution azimuthally around M87
in Virgo, confined to the northern part of the cluster (see top panel of
Fig. 1). N-body simulations of infalling galaxy groups support their
particular phase-space distribution as the result of a galaxy group
accretion on to Virgo. According to simulations, this accretion event
has taken place 2 to 3 Gyr ago, along the observer’s line-of-sight
(Vijayaraghavan, Gallagher & Ricker 2015). Such a picture receives
further support by studies of Pasquali et al. (2019) and Smith et al.
(2019), who showed that cluster galaxies occupy different zones in
the phase-space diagram with respect to their average infall time to
cluster. Based on the phase-space locus of these nine dEs (coloured
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squares in the lower panel of Fig. 1), it can be clearly seen that they
all share similar, recent infall times to Virgo. Since members of this
in-falling group are in the initial phase of accretion, the assembly
history and dynamical characteristics of their previous environment
(i.e. the parent group) should still be preserved in their kinematics
and stellar populations. This newly accreted group of dEs to Virgo
provides a unique opportunity for investigating the role of a cluster
environment in the internal dynamical evolution of dE progenitors,
particularly during the early stages of accretion. In addition to the
Virgo cluster, environmental mechanisms in their previous host halo
(such as ram pressure stripping, tidal interactions, and starvation)
have possibly affected the evolution of these nine dEs before their
accretion on to Virgo. This represents the condition that is commonly
known as pre-processing (e.g. Gallagher & Hunter 1989; Fujita 2004;
Mihos 2004; Joshi, Wadsley & Parker 2017; Han et al. 2018; Joshi
et al. 2019). To this end, studying this group of dEs can also address
the role of pre-processing in shaping present-day properties of the
‘typical’ dE population in clusters, as suggested by Toloba et al.
(2014) and Sybilska et al. (2017).
Motivated by this, we obtained IFU (Integral Field Units) data for
the nine dEs in this group using the MUSE (Multi-Unit Spectroscopic
Explorer) instrument at the Very Large Telescope (VLT). We have
performed an extensive analysis of the kinematics, dynamics, and
stellar populations of this sample. The results will be presented in a
series of papers. In this first paper, we present our kinematic analysis
of this sample of nine dEs.
This paper is organized as follows: In Section 2, we describe our
data set. In Section 3 the methods and approaches that are used in this
investigation are discussed in detail. Section 4 shows the kinematic
maps and the specific angular momentum (λR) profile of each dE in
this sample. In this section we also compare the kinematic properties
of our sample with other dEs in Virgo and low-mass galaxies in the
field. We discuss our results in Section 5. Our conclusions follow in
Section 6.
2 O B S E RVAT I O N S A N D DATA R E D U C T I O N
Our sample consists of nine dEs of a galaxy group that was accreted
recently to the Virgo cluster, discovered by Lisker et al. (2018). The
name of each target, its type, coordinates, recession velocity, effective
radii from r-band photometry images, r-band absolute magnitude,
colour, ellipticity, and total exposure time (TET), are summarized in
Table 1. All the values are taken from Lisker et al. (2006a), except
for the recession velocity which are taken from SDSS DR15 Aguado
et al. (2019). The reported values of mr (r-band apparent magnitude)
and colour were corrected for Galactic extinction by Lisker et al.
(2006a) and Janz & Lisker (2008, 2009). We estimate the stellar
mass of our sample of dEs to be 8.9 ≤ log(M [M]) ≤ 9.2 (also
reported in Table 1), based on the colour-to-(M/L) conversion (M
and L denote mass and light of system, respectively) introduced
by Bell et al. (2003) and (g−r) colours (reported in Table 1) using
a distance modulus of m–M = 31.09 mag, where m denotes the
apparent and M the absolute magnitude.
In Fig. 1 we show the spatial distribution of these nine dEs (top
panel) as well as their location in the projected phase-space diagram
(lower panel). In the top panel, other Virgo dEs with similar r-band
absolute magnitude are shown with grey circles (Lisker et al. 2006a;
Janz & Lisker 2008, 2009). The positions of four giant elliptical
galaxies in Virgo are marked with black crosses. We also overplotted
the distribution of our comparison sample from R14 and T15 in both
panels, marked with colour-coded circles and stars, respectively. The
colour bars show the values of the specific angular momentum within
one effective radius (see Section 4.3). In the lower panel, dash-dotted
lines distinguish different zones of different average infall times in the
observed phase-space diagram, which were adapted from Pasquali
et al. (2019) and Smith et al. (2019). The position of all dEs, their
projected distances, and their line-of-sight velocities are from Lisker
et al. (2006a). The virial radius of Virgo adopted here is 1.55 Mpc
(McLaughlin 1999).
The targets were observed with the MUSE following a science ver-
ification proposal in the period of 2016 December to 2017 February
and 2018 February to July (P98, ESO programmes 098.B-0619 and
0100.B-0573; PI: Lisker). MUSE is an integral-field spectrograph
mounted on VLT of the European Southern Observatory, located on
Paranal, Chile. MUSE consists of 24 spectrographs (IFUs). With
a pixel scale of 0.2 arcsec pixel−1, this instrument provides a field
of view (FOV) of 1 arcmin × 1 arcmin in wide-field mode (WFM).
MUSE delivers 90 000 sampled spectra with a resolving power of
R =3000 and a final sampling of 1.25 Å pix−1 over the range of
4500–9300 Å. MUSE’s average instrumental resolution has a full
width at half-maximum (FWHM) = 2.51 Å (Bacon et al. 2010).
During the observing runs the average seeing was nearly constant
with an FWHM of about 1.6 arcsec. The reduction of the data and
the construction of the final calibrated data cubes were carried
out within the ESO REFLEX environment (Freudling et al. 2013)
using the standard MUSE DATA REDUCTION Software (version 2.4.2)
(Weilbacher et al. 2012; Weilbacher, Streicher & Palsa 2016). Sky
residuals were removed from the final data cube by using MUSE-ZAP
(v.2.2) (Soto et al. 2016).
In this paper, we will compare the kinematic properties of our
sample of dEs with two available samples in the literature. Our Virgo
control sample consists of nine dEs investigated by Rys´ et al. (2014,
hereafter R14 sample) and 21 dEs investigated by Toloba et al. (2015,
hereafter T15 sample). While the original sample of T15 consists of
39 dEs, we only selected those that are consistent with the mass range
of our sample. The galaxies of our field control sample (hereafter
CALIFA field galaxies) are part of the CALIFA sample (Sa´nchez
et al. 2012; Walcher et al. 2014; Falco´n-Barroso et al. 2017a) with
M < 5 × 109 M, consistent with the stellar mass range of the dEs
in our sample and of the R14 and T15 sample (for more information
see Appendix A). Our field control sample consists of late-type spiral
and irregular galaxies.
3 ME T H O D S
3.1 Measurements of stellar kinematics
Our sample consists of low-surface-brightness objects. To conduct
accurate measurements of their kinematic properties, the signal-to-
noise ratio (SNR) in each data cube needed to be increased to a
minimum threshold. While considering only spaxels with SNR >3,
we increased the final SNR for each galaxy by binning spaxels
through the Voronoi binning method developed by Cappellari &
Copin (2003). This routine is an optimized fitting algorithm that
spatially bins data in a way such that each final bin achieves the
required SNR. While setting low values for the target SNR can
affect the quality of the fits, increasing it to higher values results
in bigger bins and, consequently, in a loss of spatial information,
especially in the outskirts of a galaxy. After fine-tuning based on
the characteristics of our data set (surface brightness and size of the
dEs investigated in this study), we found a minimum SNR of 40
to provide a better compromise between the spatial sampling of the
galaxy outskirts and the accuracy of the measured kinematics. The
spectrum assigned to each bin is the averaged spectrum of all spaxels
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Dwarf kinematics in a group accreted to Virgo 1907
Table 1. List of targets.
Object type α (J2000) δ (J2000) Vrecession a (km s−1) Re b (arcsec) Mr b (mag) g − ra (mag) b log (M [M]) TET (h)
VCC 0170 dE(bc) 12 15 56.30 +14 25 59.2 1415.0 31.′′57 −17.62 0.59 0.34 9.14 4
VCC 0407 dE(di) 12 20 18.80 +09 32 43.1 1876.7 18.′′38 −17.37 0.61 0.43 9.06 2
VCC 0608 dE(di) 12 23 01.70 +15 54 20.2 1819.7 25.′′77 −17.58 0.60 0.35 9.14 5
VCC 0794 dE(nN) 12 25 21.61 +16 25 46.9 1672.8 37.′′33 −17.29 0.61 0.65 9.02 3
VCC 0990 dE(di) 12 27 16.93 +16 01 28.1 1717.8 10.′′31 −17.43 0.62 0.30 9.10 3
VCC 1833 – 12 40 19.70 +15 56 07.1 1705.8 8.′′52 −17.44 0.61 0.19 9.09 1.5
VCC 1836 dE(di) 12 40 19.50 +14 42 54.0 2002.6 42.′′27 −17.45 0.58 0.66 9.06 4
VCC 1896 dE(di) 12 41 54.60 +09 35 04.9 1885.7 14.′′98 −17.04 0.62 0.05 8.94 3
VCC 2019 dE(di) 12 45 20.40 +13 41 34.1 1819.7 18.′′60 −17.65 0.63 0.22 9.16 2
Notes. Columns are: Name of target, morphological type, RA and DEC, recession velocity (Vrecession), effective radius (Re) (half-light major axis), r-band absolute
magnitude, g − r colour measured at 1Re, ellipticity at 1Re, stellar mass (M), total exposure time (TET).
a SDSS DR15 (Aguado et al. 2019), b Lisker et al. (2006a), Lisker et al. (2007).
within the defined bin. For a better accuracy of the fits, sky residuals
particularly in the red part of the spectrum are masked. Later on, the
stellar kinematics (i.e. rotational velocity and the velocity dispersion
of the stellar component) of each galaxy were computed by fitting
its binned spectra between 475 and 960 nm.
For the fitting, we utilized the Extended MILES library (E-MILES)
based on BaSTI isochrones (Pietrinferni et al. 2004). The E-MILES
spectral library has moderately high resolution in the range of 1680–
50 000 Å covering a relatively large range of ages (53 values, from
30 Myr to 14 Gyr) and metallicities (12 [M/H] values from −2.27
to +0.4). The spectral resolution of E-MILES is constant with a
FWHM ∼2.51 Å (Vazdekis et al. 2010, 2016). In Appendix B we
discuss how the resolution of different spectral libraries affects our
velocity measurements.
The E-MILES library was fitted to each bin’s averaged spectrum
through the penalized pixel-fitting algorithm (pPXF) introduced by
Cappellari & Emsellem (2004) and Cappellari (2017). PPXF derives
the line-of-sight velocity and velocity dispersion, parametrized via
Gauss–Hermite moments, by using an approach of maximum penal-
ized likelihood. In this study, the continuum slope was determined
using additive Legendre polynomials available in PPXF with a degree
of 6. As an example, in Fig. 2 we show the spectrum of a central bin
in VCC0990. The plotted spectrum was chosen to show the quality
of the data reduction and fits, as well as the degree of telluric lines
contamination in the central regions of each dE in our data set.
We obtained error estimates for each bin’s averaged spectrum by
using Monte Carlo simulations. We ran a loop of 50 realizations
for each spectrum. In each loop, we created a simulated spectrum
from the original one by adding the fit residuals, randomly reshuffled
among different wavelengths, to the original flux. The final reported
error of each bin is the standard deviation of the velocity values
obtained from the 50 realization loops. We present the error maps of
velocity and velocity dispersion for each dE in Appendix C.
4 R ESULTS
4.1 Stellar kinematics
For all of the targeted dEs, except VCC0170, the maps of stellar
velocity and velocity dispersion along the line of sight are shown in
Figs 3 and 4. Here, the left-hand panels show the stacked data-cubes,
obtained by averaging the flux across the full spectral range. For each
image, isophotes (overplotted in solid black lines) were measured by
using the ELLIPSE method by Jedrzejewski (1987). The middle panels
present the stellar velocity map of each dE, corrected for the galaxy’s
systemic radial velocity as measured by averaging the line-of-sight
velocity of its bins within 0.1 Re (to avoid offsets due to rotation of
the galaxy in the outer regions). This radial velocity is reported in
Table 2 and is in agreement with what was measured in SDSS DR15
(Aguado et al. 2019). For each dE, the maximum rotation velocity
and velocity dispersion at 0.5 Re are reported in Table 2.
We find that the stellar component in our sample of dEs shows ro-
tation with Vmax ranging from ≈5 km s−1 in VCC 1833 to ≈37 km s−1
in VCC1836.
Lisker et al. (2006a) reported the likely presence of an inclined disc
within VCC 0990 and VCC 0407. These objects show a relatively
higher rotation with V ≥30 km s−1, consistent with the value obtained
with long-slit spectroscopy by Toloba et al. (2015).
The velocity dispersion map of each dE is shown in the right-
hand column of Figs 3 and 4. As shown in the maps, the velocity
dispersion profile of our sample of dEs has a shallow gradient with
20 ≤ σ ≤ 35 km s−1 within the field of view. A velocity dispersion
gradient, although not statistically significant, is observed in VCC
1836, rising from 27.5 ± 3 km s−1 in the central regions to more than
40.0 ± 18.0 km s−1 in the outskirts of the galaxy.
The stellar kinematics of VCC0170 was measured following the
same steps as for the other dEs in our sample after masking the gas
emission lines in its central region (Section 4.2). The velocity map
of VCC 0170 shows rotation with Vmax ≈ 18 km s−1 in the outskirts
of the galaxy. Like other members of our sample, VCC0170 has a
flat velocity dispersion as a function of radius. The stacked image
from the MUSE cube, the velocity map, and velocity dispersion map
of VCC0170 are presented in the lower panels of Fig. 5.
The position angle of the photometric major axis (PAphot) is
overplotted with a solid black line on the velocity map of each galaxy.
For each of our dEs we retrieved the PAphot values measured in the
g, r, and i bands from SDSS DR15 (Alam et al. 2015). They were
obtained by fitting an exponential profile to the galaxy’s observed
surface brightness profile. For each galaxy we averaged the available
PA values and computed the corresponding standard deviation. They
are listed in Table 2. On the same maps, the kinematic position
angle (PAkin) of each galaxy is shown with a black dashed line. We
measured the PAkin using the kinematic maps of each dE following
the method of Krajnovic´ et al. (2006). To better quantify any offset,
we computed the kinematic misalignment angle  defined by Franx,
Illingworth & de Zeeuw (1991):
sin  =| sin(PAphot − PAkin) | . (1)
The resulting values are listed in Table 2.
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Figure 2. Example of a PPXF fit of a central bin of VCC0990. The top panel represents the full range of the observed spectrum, while in the lower panels,
a zoomed-in view is provided for a better illustration of the fitting accuracy and high quality of the data. In all the panels, the observed spectrum is shown in
black, while the best PPXF fit is plotted in red. The residuals between the observed spectrum and its best fit are plotted in orange shifted up by 0.5 for legibility.
VCC1896 has the highest misalignment between its photometric
and kinematic position angles ( = 82.7 ± 3.1) and shows prolate-
like rotation, with Vmax ≈ 9.3 kms−1 and σ ≈ 23.0 kms−1 consistent
with what was measured by Penny et al. (2015).
VCC0608 is another member of our sample that shows high
kinematic misalignment ( = 42.8 ± 12.5). Lisker et al. (2006a)
reported a possible disc substructure within this galaxy. VCC0608
also exhibits an overall boxy shape in the image constructed from its
MUSE data cube. We will discuss these two cases in more detail in
Section 5.2.
Several studies of the Virgo cluster report the existence of the
so-called kinematically decoupled cores (KDC) in early-type dwarf
galaxies (e.g. see Geha et al. 2003; Koleva et al. 2011; Rys´ et al. 2013;
Toloba et al. 2014; Gue´rou et al. 2015). Observations of Toloba et al.
(2014) show that about six per cent of dEs in the Virgo cluster host
a KDC. However, we do not detect any KDC in our sample of nine
Virgo dEs. Although our sample is not statistically significant, thus
not representative, the zero detection in this study is consistent with
the results of Toloba et al. (2014).
4.2 Gas emission lines in VCC 0170
Except for VCC0170 and VCC1836, our sample of dEs lacks nebular
emission lines. In the case of VCC1836, the observed emission lines
are produced by a background galaxy at redshift z = 0.552 (see
Appendix E for a complete analysis).
The detection of nebular emission lines in the central part of
VCC0170 is consistent with its blue colour, which is likely due to the
presence of young massive stars. While this galaxy was originally
classified as a late-type galaxy by Gavazzi et al. (2005), it was
recognized as a dE member of Virgo by Lisker et al. (2006a), who
also pointed out an irregularly shaped distribution of the blue colour
in the central part of VCC0170. H I emission also is observed within
the central part of this galaxy, indicating a MH I
Mbar
ratio of less than
one per cent (Gavazzi et al. 2005; Lisker et al. 2006a,b). Here Mbar
denotes the baryonic mass.
As shown in the top right-hand panel of Fig. 5, the irregular
shape of VCC0170’s core is also visible in the MUSE stacked
image. This is the region marked with a red box in the top left-
hand panel of Fig. 5, which covers an area of 8 × 8 arcsec2
(0.40 kpc2 at an assumed distance of 16.5 Mpc). A complete list
of detected emission lines and their observed fluxes are reported
in Appendix D. Using the ‘Baldwin, Phillips, and Terlevich’ (BPT)
diagram (Baldwin, Phillips & Terlevich 1981), we confirm that the
nebular emission from this irregularly shaped region is due to star
formation. Furthermore, we have measured the gas kinematics for
VCC0170 inside the marked region in Fig. 5 by fitting a Gaussian
function to the H α line for each spaxel with SNR ≥ 3. We chose this
particular emission line since it has a higher SNR in comparison to
other available lines (see Fig. D1). The resulting gas velocity map
is shown in Fig. 6. We see that the gas component has a systematic
offset in velocity (≈40 ± 8.0 km s−1) with respect to the underlying
stellar component.
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Dwarf kinematics in a group accreted to Virgo 1909
Figure 3. Stacked MUSE images and stellar kinematic maps of our sample of dEs. In the left-hand panels, the stacked MUSE image of each galaxy is shown
with black isophotes overplotted. From the inside out, the isophotes indicate regions with a surface brightness of 20.59, 21.00, 21.85, and 22.52 mag arcsec−2
(ABmag). In the middle panels, the line-of-sight velocity map of the stellar component of each dE is plotted. The kinematic position angle (PAkin) and photometric
position angle of the major axis (PAphot) are traced with dashed and solid lines, respectively. In the right-hand panels, the associated velocity dispersion map is
shown. For a better comparison, the isophotes are drawn in all the panels.
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Figure 4. Continued.
4.3 Specific angular momentum
To parametrize the rotation in our sample of dEs and in order to
compare it with other dEs in the Virgo cluster and the CALIFA field
galaxies, we derived the specific angular momentum (λR) profile of
each dE, following Emsellem et al. (2007):
λR =
∑N
i=1 FiRi |Vi |∑N
i=1 FiRi
√
V 2i + σ 2i
, (2)
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Dwarf kinematics in a group accreted to Virgo 1911
Table 2. Kinematic parameters of our nine dEs.
Object Vrad Vmax σ0.5Re PAph PAkin  λRe λ0.5Re
(km s−1) (km s−1) (km s−1) (deg) (deg) (deg)
VCC 0170 1403.4 11.0 ± 15.0 24.5 ± 20 175.41 ± 0.51 155.2 ± 15.5 20.2 ± 15.5 0.45 0.39
VCC 0407 1881.5 30.6 ± 9.0 30.9 ± 18.0 89.56 ± 1.94 111.1 ± 3.1 20.4 ± 3.6 0.67 0.40
VCC 0608 1807.7 7.8 ± 14.0 21.6 ± 19.0 137.1 ± 1.77 180.0 ± 12.4 42.9 ± 12.5 0.38 0.21
VCC 0794 1669.0 10.2 ± 6.0 30.3 ± 20.0 161.7 ± 0.42 167.6 ± 3.1 5.9 ± 3.1 0.48 0.27
VCC 0990 1715.1 10.5 ± 4.0 31.5 ± 6.0 134.5 ± 0.24 133.8 ± 4.6 0.5 ± 4.6 0.27 0.24
VCC 1833 1711.4 4.9 ± 2.0 32.6 ± 4.0 172.8 ± 0.38 189.6 ± 5.0 16.7 ± 5.0 0.15 0.11
VCC 1836 1985.2 36.9 ± 7.0 34.6 ± 17.0 16.97 ± 0.18 24.8 ± 3.1 7.8 ± 3.1 0.55 0.53
VCC 1896 1872.5 9.3 ± 7.0 23.0 ± 17.0 27.5 ± 0.58 110.2 ± 3.1 82.7 ± 3.1 0.22 0.23
VCC 2019 1822.1 17.3 ± 6.0 30.9 ± 15.0 153.2 ± 0.96 160.6 ± 9.3 7.2 ± 0.3 0.73 0.30
Notes. The columns contain the name of the targets, the mean systemic line-of-sight velocity within 0.1 Re, maximum velocity (Vmax), and
velocity dispersion (σ ) at 0.5 Re, the position angle of the photometric major axis (PAph) from Alam et al. (2015), the kinematic position
angle (PAkin), the kinematic misalignment angle (), the specific angular momentum at 1 Re (λRe) and at 0.5 Re (λ0.5Re). Please note that
values marked with a star are measured through extrapolation of the λR profile.
Figure 5. VCC0170 and its central gas component. Top panel: On the left-hand side, the MUSE stacked image is presented where the central region is marked
with a red box, 8 × 8 arcsec2 in size. In the right-hand panel, a zoomed-in image of this particular region is provided, where the irregular shape of the core
can easily be recognized. Bottom panels: The kinematic maps of the stellar component in VCC0170. The left-hand panel shows the MUSE stacked image
while the rotation velocity and velocity dispersion of stellar component are plotted in the middle and right-hand panels, respectively. Isophotes, kinematic, and
photometric position angles are also overplotted as in Fig. 3.
where Ri, Fi, Vi, and σ i are galactocentric distance, average flux, ro-
tational velocity, and velocity dispersion of each bin, respectively. Ri
is the distance of each bin to the photometric centre of the galaxy. The
photometric centre is measured using the Multi-Gaussian Expansion
(MGE) fitting algorithm introduced by Emsellem, Monnet & Bacon
(1994) and adapted by Cappellari (2002).
The resulting λR profiles are shown in the left-hand panel of Fig. 7
with different shades of blue and red. For a better comparison and in
the same panel, the range of λR profiles of dEs in the R14 sample is
also shown by a red-shaded region. Similarly, the range of λR profiles
of the CALIFA field galaxies is shown by a blue-shaded region. Their
λR profiles were measured by Falco´n-Barroso et al. (2019).
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Figure 6. The gas velocity map from the H α emission line observed in the
central part of VCC0170. This region is marked in the top right-hand panel
of Fig. 5. The gas velocity here is relative to the stellar velocity.
In the left-hand panel of Fig. 7, five dEs in our sample, namely
VCC0170, VCC0407, VCC0794, VCC1836, and VCC2019, show
λR profiles similar to those of the CALIFA field galaxies. We trace
their λR profiles with different shades of blue. The remaining four
dEs in our sample, namely VCC0608, VCC0990, VCC1833, and
VCC1896, exhibit λR profiles comparable with the R14 sample (the
red region), hence they are traced with different shades of red.
Although the dEs investigated in this study have quite similar
stellar masses, the large spread in the values and slopes of their
λR profiles is noticeable. VCC0170, VCC0407, and VCC2019 are
characterized by steeper profiles that rise from λR = 0.06 at the
centre to λR = 0.6 at about 1 Re. The λR profile of VCC1836 shows a
truncation at about 0.40 Re. This galaxy is the most extended member
of our sample and the MUSE field of view is too small for sampling it
to 1 Re. The same explanation applies to the λR profiles of VCC0170
and VCC0794, which only reach 0.60 Re and 0.50 Re, respectively.
In the right-hand panel of Fig. 7, we plot the λRe– diagram (λRe
versus ellipticity) for the same set of galaxies as shown in the left-
hand panel. Here λRe is the value of the specific angular momentum
at one effective radius. In this diagram, the dEs investigated in this
study are denoted with squares and are colour-coded as in the left-
hand panel of the same figure. dEs in the R14 sample and the CALIFA
field galaxies are denoted with pink and blue circles, respectively. As
mentioned earlier, three dEs in our sample are too extended for the
MUSE field of view to be mapped up to 1 Re. For these three galaxies,
the λRe value at 1 Re is measured through extrapolation of their λR
profiles. The obtained values of λR at 1 Re and 0.5 Re are reported in
Table 2. The black dashed line in Fig. 7 corresponds to (0.31 × √),
which is the threshold separating slow from fast-rotating galaxies,
according to Emsellem et al. (2011). The λRe– diagram indicates
that, at a fixed stellar mass, on average the CALIFA field galaxies
show higher values of λR and ellipticity in comparison to dEs in
Virgo. The low λR dEs in our sample are mostly distributed close
to the boundary separating the fast- from the slow-rotating galaxies.
As mentioned in Section 2, our sample of dEs are located in the
outskirts of the Virgo cluster and according to the right-hand panel
of Fig. 7, they are fast-rotating systems. This is consistent with the
results of Boselli et al. (2014) who showed that fast-rotating galaxies
are mostly observed in the outskirts of the Virgo cluster.
5 D ISCUSSION
5.1 Rotation in dEs
The kinematic maps of our sample of dEs show different degrees of
rotation (Figs 3 and 4). This result is consistent with other studies
of Virgo dEs in the literature. Toloba et al. (2011, hereafter T11)
investigated a sample of 21 dEs in Virgo with −15 ≥ Mr > −18 for
which they measured a similar range of rotational velocities, from 0
to 50 km s−1. The velocity dispersion of the T11 sample also varies
between 30 and 50 km s−1. Meanwhile, by focusing on a smaller
sample of Virgo dEs with a similar range of Mr, R14 investigated
12 dEs and showed that their rotational velocity varies from 0 to
40 km s−1. The velocity dispersion maps of dEs in that study also
show a flat radial gradient, with values ranging from 40 to 60 km s−1.
The values of rotational velocity for the dEs investigated in this study
are in the same range as in T11 and R14 but skewed towards higher
values, especially at higher galactocentric distances.
To compare the physical properties of dEs with the aim of
investigating the role of the environment in their transformation,
one needs to consider their infall time (i.e. how long a dE was
exposed to environmental effects). That is mainly due to the fact
that environmentally induced mechanisms such as tidal interactions
and ram pressure stripping occur over different but not short time-
scales (Boselli & Gavazzi 2006; Tinker & Wetzel 2010; Bialas
et al. 2015). According to Boselli & Gavazzi (2006), ram pressure
stripping occurs with a typical time-scale of less than 1 Gyr and tidal
interactions happen on a time-scale of ∼ 2 Gyr, which corresponds to
the crossing time of the Virgo cluster. Average values of infall time
can be derived using a galaxy’s position in the projected phase-space
diagram of their host halo (Rhee et al. 2017; Pasquali et al. 2019;
Smith et al. 2019). Based on their position in the observer’s phase-
space (see the lower panel of Fig. 1), we know that our sample dEs
are in their initial phase of accretion to the Virgo cluster, i.e. they
were accreted to Virgo around 2–3 Gyr ago (Vijayaraghavan et al.
2015; Lisker et al. 2018). On the other hand, the position of the R14
and T15 dE sample in the observer’s phase-space diagram reveals
that around 60 per cent of them had an earlier average infall time than
our sample of dEs. The difference in accretion time may explain the
overall higher rotational velocity of our sample dEs, in comparison
to the population investigated by R14. We notice here that, when
inferring the average infall times from the projected phase-space
diagram of galaxy clusters, Pasquali et al. (2019) consider accretion
to the whole cluster and not infall on to its dominant structures or
substructures. The latter case would possibly deliver more accurate
infall times in the case of the Virgo cluster, which is noticeably
structured. Thus, the comparison of infall time of the different dE
samples considered in this paper should be taken with some caution.
Furthermore, results of Falco´n-Barroso et al. (2017b) for the CAL-
IFA field galaxies show higher values of rotational velocity (from 20
to 100 km s−1) and velocity dispersion (from 20 to 90 km s−1). Our
sample of dEs show lower degrees of rotation in comparison to
the CALIFA field galaxies. As already discussed in Section 1, such
differences between the kinematics of low-mass field galaxies and
Virgo dEs can be expected due to the cluster’s role in transforming its
accreted members. Besides, prior to their infall to Virgo, our sample
of dEs belonged to a rather massive group which models predict to
have been as massive as 1013 M (Lisker et al. 2018). Therefore, the
lower degrees of rotation observed in our sample dEs in comparison
to the CALIFA field galaxies, may be understood in light of pre-
processing in their parent group. This will be discussed with more
detail in Section 5.3.
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Dwarf kinematics in a group accreted to Virgo 1913
Figure 7. Left-hand panel: The specific angular momentum profiles. The red-shaded area shows the minimum-to-maximum range of the λR profiles of dEs in
the sample. The blue-shaded region represents the minimum-to-maximum range of the λR profiles of the CALIFA field galaxies. dEs in our sample with low
λR profiles similar to the R14 sample are traced with lines of different shades of red. Profiles of those dEs overlapping with the CALIFA galaxies are traced
with lines of different shades of blue. Right-hand panel: λRe – diagram distribution for our sample of dEs (colour-coded squares), dEs in the R14 sample (red
circles) and ten CALIFA field galaxies from Falco´n-Barroso et al. (2019) (blue circles). The dashed black line corresponds to the separating threshold between
slow- and fast-rotators (0.31 × √) introduced by Emsellem et al. (2011).
5.2 Stellar rotation in VCC0608 and VCC1896
Among the dEs investigated in this study, VCC0608 and VCC1896
present a rather high offset between their photometric and kinematic
position angles. These two dEs cannot be classified as regular rota-
tors. In this subsection we will discuss different possible explanations
for the presence of such an irregular kinematic structure.
VCC1896 with  = 82.7◦ ± 3.1◦ has the highest offset between
its photometric and kinematic axis. Taken at face value, VCC1896
would look like a dE with minor axis rotation. Lisker et al. (2006a),
Lisker, Brunngra¨ber & Grebel (2009) reported the existence of two
rather faint spiral arms for VCC1896 and a bar aligned with the outer
photometric minor axis of this galaxy. Mastropietro et al. (2005)
showed that, due to tidal interactions, asymmetric features and open
spiral arms can be formed in those dEs which are located in the
outskirts of cluster. In addition to that and as shown by Kwak et al.
(2019), the cluster tidal field can trigger the formation of weak spiral
arms in disc dEs. These results might explain the existence of the
faint spiral arms in VCC1896. Due to their low surface brightness,
the faint spiral arms of VCC1896 are not detected in our MUSE data
cube; therefore, they are not present in the kinematic maps of this dE.
We only detected the bright bar of this dE in its MUSE cube. While
the presence of the bar explains the high offset between kinematic
and photometric PA, it may also be a morphological feature relevant
to the evolution of VCC1896.
The bar in the central region of VCC1896 can either be a pristine
structure that formed and developed through standard processes
or a tidal feature that formed as a result of interactions with the
environment (e.g. see Mastropietro et al. 2005). Tidal interactions
are known as one of the most dominant mechanisms that can induce
instabilities in the stellar disc of galaxies and form bars, particularly
in central regions of galaxy clusters and groups (Łokas et al. 2016).
Moreover, Kwak et al. (2019) showed that tidal effects due to galaxy–
galaxy interactions, which are more dominant in the central parts of
a cluster, can be efficient in causing instabilities to bar formation
in dwarf galaxies in a Virgo-like halo. As for Virgo, observations of
Janz et al. (2012) also confirm that dEs with bars are mostly observed
in the central parts of the cluster. Also, simulations have shown that
bar instabilities can be triggered in Local Group dwarf galaxies after
their first pericentre passage (Gajda, Łokas & Athanassoula 2016).
However, based on Pasquali et al. (2019), the probability of having
already experienced a first pericentre passage for galaxies with a
similar average infall time as VCC1896 is not particularly high. Thus,
the presence of a bar structure in VCC1896 may be interpreted in the
light of pre-processing in its parent group, and less likely in Virgo.
VCC0608 with  = 42.9◦ ± 12.5◦ is the second dE in our
sample showing significant misalignment between its kinematic and
photometric minor axis. The observed kinematic misalignment can
be the trace of past or on-going tidal interactions with the massive
halo of Messier 100 (projected distance to M100 ∼ 28.7 kpc at an
assumed Virgo distance of 16.5 Mpc). VCC0608 is a boxy-shaped
galaxy as its isophotes on the MUSE stacked image show A4/a ≈
−0.02 ± 0.003 at 1 Re. Therefore, VCC0608 shares similarities
with LEDA 074886 (Graham et al. 2012), which is a boxy-shaped
dwarf galaxy with M ≈ 109 M in the halo of NGC 1407. Following
the same argument as in Graham et al. (2012), the boxy shape of
VCC0608 and its high kinematic misalignment angle may indicate
that this galaxy is the remnant of a past nearly edge-on merger of
two disc dwarf galaxies (see e.g. Naab, Jesseit & Burkert 2006). In
this case, the inner disc of the galaxy is believed to have formed from
gas driven inward during the merger event (Graham et al. 2012). In
that case, a younger stellar population is expected to be observed
in the central region of VCC0608. This scenario will be tested in a
forthcoming paper through the analysis of the stellar populations of
our sample of dEs.
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5.3 Group versus cluster environment
Based on the kinematic analysis performed in this study, we found
that the specific angular momentum (λR) profile of our sample of dEs
is intermediate between that of dEs in Virgo and CALIFA low-mass
galaxies in the field.
The angular momentum is often described as an essential metric
for understanding the evolution of low-mass galaxies as it may
reflect conditions of their early environments where these objects
formed or spent most of their lives (see e.g. Shi, Wang & Mo 2015).
The angular momentum is typically parametrized in terms of the
projected stellar specific angular momentum along the line of sight
(λR). This parameter is weighted according to the luminosity, thus
it is considered to be a relevant proxy for the real stellar specific
angular momentum of a galaxy (Emsellem et al. 2007, 2011; Jesseit
et al. 2009). Studies have revealed that, through the lifetime of a
galaxy, mechanisms such as gas accretion and star formation can
effectively increase the angular momentum of the entire structure.
For instance, Zoldan et al. (2019) showed that at a fixed stellar
mass, gas-rich systems tend to have higher degrees of rotation.
Meanwhile, other studies show that mergers and tidal heating of
the disc can decrease the net rotation of a galaxy and thus its
angular momentum (for example, see van den Bosch, Burkert &
Swaters 2001; Brook et al. 2011; Naab et al. 2014; Penoyre et al.
2017; Greene et al. 2018). Tidal interactions occur frequently in
galaxy groups or central parts of a galaxy cluster, mostly due to
interactions with the halo’s tidal field and encounters with other
cluster members. Tidal interactions are known to be one of the effects
responsible for the evolution of λR as well as for the morphological
transformation of galaxies in clusters and groups (e.g. Boselli &
Gavazzi 2006; Lisker et al. 2006a; Lisker et al. 2007; Michielsen
et al. 2008; Kormendy et al. 2009; Lisker et al. 2009; Toloba et al.
2009; Geha et al. 2010; Toloba et al. 2011; Janz et al. 2012; Toloba
et al. 2012; Rys´ et al. 2014). On the other hand, ram pressure stripping
is expected to have a marginal effect on altering the λR profile of a
galaxy (Yozin & Bekki 2016). Following the scenario suggested by
R14, cluster low-mass galaxies with earlier infall time (i.e. with more
pericentre passages) are expected to experience a more significant
tidally driven change of λR. Accordingly, newly accreted low-mass
galaxies are expected to exhibit internal kinematics intermediate
between that of low-mass field galaxies and ‘more ancient infallers’
in cluster. Such expected intermediate kinematics is shown in the
left-hand panel of Fig. 7. In this plot, our sample dEs, with an
average infall time of 2–3 Gyr to Virgo, distribute between the
CALIFA low-mass galaxies in the field and the Virgo dEs in the
R14 sample. The latter sample has been exposed to environmental
effects for longer time (∼1–2 Gyr), according to the lower panel of
Fig. 1.
Along with λR, the ellipticity of a satellite galaxy is also expected
to decrease with infall time on the ground of tidally driven effects that
can thicken the stellar disc of a system. For instance, D’Eugenio et al.
(2015) show that in a rich cluster, the fraction of spheroidal early-
type galaxies increases towards lower cluster-centric distances (but
see: Weijmans et al. 2014). In Fig. 8 we represented the same λRe–
diagram of Fig. 7 where different groups of galaxies are now colour-
coded according to their average infall time, as read from the bottom
panel of Fig. 1. dEs with high and low values of λRe in our sample are
marked with squares and crosses, respectively. The R14 dE sample is
denoted by circles and, additionally, the T15 sample is indicated with
stars. In Fig. 8, the CALIFA field galaxies (denoted with triangles)
with an assigned average infall time of zero Gyr exhibit, on average,
higher ellipticity than Virgo dEs. The average ellipticity and λRe of
each group are indicated by their corresponding symbol, but bigger,
Figure 8. The λRe– diagram colour coded based on the average infall
time of each galaxy. The CALIFA field galaxies are denoted with black
triangles. Our sample of dEs with high and low values of λRe are traced
with squares and crosses, respectively. The R14 sample dEs are marked with
circles. The T15 sample dEs are marked with stars. The median ellipticity
and λRe of each group of galaxies are also denoted with their corresponding
symbol, but bigger, and error bars. The dashed black line corresponds to the
separating threshold between slow- and fast-rotators (0.31 × √) introduced
by Emsellem et al. (2011).
and error bars. A spread, similar to that of the λR profiles, is also
visible in the ellipticity of the nine dEs investigated in this study. The
ellipticity of the low-λR dEs in our sample is comparable to those dEs
in the T15 sample of similar infall times. The ellipticity of low-λR
dEs in our sample is also comparable to those dEs in the R14 sample
that have, on average, earlier infall times.
Given their similar stellar masses and comparatively short infall
time to Virgo, the spread in the λR profile of our sample of dEs
is rather large (Fig. 7). One possible explanation is that the Virgo
environment has had only marginal impact on the evolution of the λR
profiles of our sample of dEs (if any) on the ground that their recent
infall time decreases their probability of having already experienced
a pericentre passage. In this regard, Pasquali et al. (2019) showed
that near 40 per cent of satellite galaxies with an average infall
time of 3.6 Gyr have not experienced their first pericentre passage
yet. Environmental processes during pericentre passages are more
efficient in altering λR rather than at higher cluster-centric distances
(Villalobos et al. 2012).
According to the scenario proposed by Lisker et al. (2018), prior
to their infall to Virgo, our sample of dEs were probably members
of a rather massive galaxy group (1013 M) where the probability
of tidal interactions is high (Tal et al. 2009) due to lower velocity
dispersion of the halo (Boselli & Gavazzi 2006). In fact, Villalobos
et al. (2012) have shown that in an equally massive halo, the λR profile
of a galaxy progressively flattens and decreases in value as the galaxy
undergoes more and more pericentre passages. The latter is mainly
by virtue of substantial dynamical disturbances that arise often after
several orbital periods (i.e. >4 Gyr). In this regard, the diversity in
ellipticity and λR profiles of our sample of dEs may be a reflection
of different infall times in their previous host halo. Hence, the four
dEs with λR profiles as low as those of the Virgo dEs in the R14
sample (in red in Fig. 7) may have been accreted earlier on to their
previous host halo, and therefore evolved more strongly under their
group’s environmental mechanisms. They include two galaxies with
footprints of tidal interactions in their kinematics (i.e. VCC1896 and
VCC0608 in Section 5.2). Contrarily, the remaining five dEs with λR
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Dwarf kinematics in a group accreted to Virgo 1915
profiles similar to those of the CALIFA field galaxies (blue profiles
in Fig. 7) may have experienced partial transformation mainly due
to their comparatively lower number of pericentre passages in this
previous host halo, if any. In this picture, what we have measured for
our sample of dEs may be considered as the result of pre-processing
in a different environment than Virgo.
Stripping the gas content of galaxies is a known effect of high-
density environments (i.e. massive galaxy groups and clusters)
mainly due to ram pressure stripping (Boselli & Gavazzi 2006;
Boselli et al. 2008). The efficiency of such mechanism depends
directly on the density of the inter-cluster medium, as well as on
the speed and orbit of the accreted galaxy (e.g. Gunn & Gott 1972;
Vollmer et al. 2001; Steyrleithner, Hensler & Boselli 2020). Ram
pressure stripping can be noticeably efficient for low-mass galaxies,
mainly due to their shallow potential well. According to Boselli et al.
(2008, 2014), low-mass systems, if gas rich, can become quiescent
systems on rather short time-scales in the Virgo cluster. Thus, both
the previous parent group and the Virgo cluster could have been
responsible for gas stripping in our sample of dEs (for gas stripping
in groups see Brown et al. 2017). Moreover, McPartland et al. (2016)
and Ebeling & Kalita (2019) show that the efficiency of ram pressure
stripping is highest when the velocity vectors of galaxies in a group
being accreted on to a cluster are aligned, thus add up to the velocity
vector of the infalling group itself.
Ram pressure can perturb and make the gas content of galaxies un-
stable (Kronberger et al. 2008; Steyrleithner et al. 2020). We observe
these effects in VCC0170, whose gas component exhibits a velocity
offset from the galaxy’s stellar body, and nebular emission lines
consistent with recent/on-going star formation (see Appendix D)
(Roediger et al. 2014). This finding allows us to envisage an
alternative scenario. In our sample, those dEs with higher values
of λR could have been low-mass, late-type, star-forming galaxies in
the previous host halo, which lost most of their gas reservoir after
infall to Virgo, mostly via ram pressure stripping (Toloba et al. 2009;
Boselli et al. 2014). As already explained in the first scenario, tidal
interactions only develop during several orbital periods, comparable
to the cluster dynamical time-scale. Therefore, our sampled dEs with
high but comparable values of λR and ellipticity to field galaxies have
experienced little dynamical transformation thus far in Virgo. In this
picture, the low-λR dEs in our sample could have been accreted to
Virgo as early-type galaxies that likely lost their gas content and were
transformed dynamically in their previous environment. Moreover,
tidal disturbances (such as in VCC0608 and VCC1896) indicate
tidal encounters that occurred in the past, but possibly not in Virgo,
due to their rather recent infall time. We then suggest that the low-
λR dEs in our sample have likely been processed by their previous
group in both the first and second scenarios. This second scenario is
more in line with what suggested by Toloba et al. (2009, 2011, 2015).
Further analysis on age, metallicity, and star formation history of this
sample can provide us with a better understanding of their evolution
before and after accretion onto the Virgo cluster and thus evaluate
this scenario. The results of such an analysis will be presented in a
forthcoming paper.
Alternatively, the observed spread in the λR profiles and ellipticity
of our sample could be consistent with a recently accreted population
of field dEs, with intrinsically different degrees of rotation and
ellipticity. Janz et al. (2017) studied a sample of nine isolated
quenched dEs in the Local Volume and reported diverse kinematics
for this sample. In this regard, the four low-λR dEs in our sample could
originally have been primordial slow-rotating objects with a rather
flat λR profile even before entering a high-density environment. The
latter is consistent with the results of Wheeler et al. (2017), who show
that in the absence of external tidal fields, isolated dwarf galaxies
can be formed as dispersion-dominated systems. In this alternative
interpretation, the dEs with higher values of λR can intrinsically be
different from their low-λR counterparts in our sample, exhibiting
kinematics due to relatively little or moderate processing thus far in
the cluster. Geha et al. (2012) showed that the fraction of field dEs
in the same stellar mass range of our sample and with no active star
formation is less than 0.2 per cent (corresponding to a total number
of four galaxies). According to this study, we cannot firmly rule out
the possibility that the low-λR dEs in our sample were also field
members accreted to Virgo. However, it would be a rare coincidence
that these four field galaxies were accreted on to Virgo with a similar
velocity as the accreted group and at the same time. We believe
that constructing a more statistically rich sample of dEs in different
environments (such as field, groups, and galaxy clusters) can provide
us with a more complete picture of the role of environment in altering
kinematics and other properties of low-mass galaxies.
6 C O N C L U S I O N S
In this study, we investigated the stellar kinematics of nine dEs
belonging to a group that was accreted on to the Virgo cluster, along
the observer’s line of sight, about 2–3 Gyr ago (Lisker et al. 2018).
Their similar accretion time has also been confirmed by the position
of these dEs in the phase-space diagram of Pasquali et al. (2019) and
Smith et al. (2019). Members in this sample provide a unique test bed
to study the evolution of dEs in their initial phases of infall to Virgo.
We obtained MUSE IFU data cubes for these nine dEs in order to
investigate their kinematics, dynamics, and stellar populations. In this
work, we present the kinematics of these nine dEs. We measured the
line-of-sight velocity and velocity dispersion across the stellar body
of each dE, using PPXF on the full spectral range delivered by MUSE.
Later on, we used these values in order to compute the λR profiles
of our sample dEs with the purpose of comparing them with other
similar measurements for different environments available from the
literature. The main results of our analysis can be summarized as
follows:
(i) The stellar component of our sample of dEs shows a rather
similar range of rotational velocities in comparison to other dEs in
Virgo (from 5 to 40 km s−1). The velocity dispersion of their stellar
component varies between 20 and 35 km s−1.
(ii) We investigated the kinematic misalignment angle of the dEs
in our sample, and we report two cases of misaligned kinematics,
namely VCC1896 and VCC0608. In the case of VCC1896, we show
that the observed offset between the kinematic and photometric
position angle is basically due to the presence of a bright bar
that dominates the photometric measurements. We also find a high
kinematics misalignment for the boxy-shaped VCC0608. Past or on-
going tidal interactions with the massive halo of M100 or a past major
merger, considering similarities between this particular dE and the
galaxy LEDA 074886, may be possible causes of the observed high
kinematic misalignment in this galaxy.
(iii) VCC0170 is the only dE in our sample that shows strong
nebular emission lines in its very central region due to star formation,
presumably triggered by recent/on-going gas stripping in Virgo.
Further analysis of the star formation history and metallicity of this
particular dE (to be presented in a forthcoming paper) may shed light
on the origin of the asymmetrical structure in the centre of VCC 0170.
(iv) The λR profile and ellipticity of the dEs investigated in this
work show, on average, intermediate values between the dEs studied
by Rys´ et al. (2014), which were on average accreted earlier onto
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Virgo, and CALIFA field galaxies of the same stellar mass (from
Falco´n-Barroso et al. 2019).
(v) Given their infall time to Virgo and similar stellar masses, the
spread in the λR profile of our nine dEs can be interpreted as follows:
the low-λR dEs were likely transformed in and by their previous
host group, prior to their infall on to Virgo, while the high-λR dEs
were only partially transformed in their previous host group. The
latter sub-group of dEs may be experiencing ram pressure stripping
in Virgo.
(vi) In Appendix E we report on the serendipitous discovery of
two intermediate redshift galaxies, found in the MUSE data cubes of
VCC0170 and VCC1836.
AC K N OW L E D G E M E N T S
We thank the referee for useful suggestions that improved the
clarity of this paper. We acknowledge financial support from the
European Union’s Horizon 2020 research and innovation program
under the Marie Sklodowska-Curie grant agreement no. 721463 to
the SUNDIAL ITN network. We would like to thank Jakob Walcher,
Jorge Iglesias, J.K Barrera-Ballesteros, D.J. Bomans, A. Miskolczi,
and B. Biskup for providing environmental information for the
CALIFA sample. BB acknowledges the support of the International
Max Planck Research School (IMPRS) for Astronomy and Cosmic
Physics at the University of Heidelberg. JFB acknowledges support
through the RAVET project by the grant AYA2016-77237-C3-1- P
from the Spanish Ministry of Science, Innovation and Universities
(MCIU) and through the IAC project TRACES which is partially
supported through the state budget and the regional budget of the
Consejerı´a de Economı´a, Industria, Comercio y Conocimiento of
the Canary Islands Autonomous Community. GvdV acknowledges
funding from the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation programme
under grant agreement No 724857 (Consolidator Grant ArcheoDyn).
DATA AVAILA BILITY
This work is mainly based on observations collected at the European
Southern Observatory under P98, ESO programmes 098.B-0619 and
0100.B-057.
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APPENDI X A : CALI FA FI ELD G ALAXI ES
The Calar Alto Legacy Integral Field Area (CALIFA) survey was
designed to investigate the evolution of galaxies through cosmic
time by a detailed spectroscopic study of ∼600 galaxies in the Local
Universe (Sa´nchez et al. 2012; Walcher et al. 2014). As our field
control sample, we selected 10 low-mass field galaxies from the
CALIFA survey. The selected CALIFA objects are star-forming
galaxies of similar stellar mass to those observed in this study,
following the criteria mentioned in Section 4.3. We summarize the
properties of these 10 galaxies in Table A1.
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Table A1. List of CALIFA targets.
Object type α (J2000) δ (J2000) Re a (arcsec) Mr (mag) M(×1010) [M]  λRe
NGC 0216 Sd 00 41 27.16 −21 02 40.82 20 −18.99 0.19 0.711 0.52
NGC 3057 Sdm 10 05 39 +80 17 12 32 −19.17 0.12 0.269 0.43
NGC 5682 Scd 14 34 44.97 +48 40 12.83 26 −19.39 0.25 0.764 0.68
NGC 7800 Ir 23 59 36.75 +14 48 25.04 32 −19.56 0.19 0.607 0.19
UGC05990 Sc 10 52 37 +34 28 58 12 −18.32 0.16 0.742 0.17
UGC08231 Sd 13 08 37.55 +54 04 27.73 19 −19.28 0.14 0.664 0.40
UGC08733 Sdm 13 48 38.99 +43 24 44.82 30 −19.75 0.26 0.437 0.46
UGC10650 Scd 17 00 14.58 +23 06 22.83 23 −19.32 0.20 0.782 0.33
UGC10796 Scd 17 16 47.72 +61 55 12.42 20 −19.56 0.28 0.416 0.24
UGC12054 Sc 22 29 32.44 +07 43 33.68 15 −18.41 0.10 0.739 0.41
Notes. Columns are: Name of target, morphological type, RA and DEC, effective radius, r-band absolute magnitude, stellar mass,
ellipticity at 1Re and specific angular momentum at 1Re. All the values were obtained from Falco´n-Barroso et al. (2019).
A PPENDIX B: INSTRUMENTA L R ESOLUTI ON
For measuring kinematics of a galaxy by fitting a stellar library, the
data and the template library should have the same velocity scale.
The latter is possible through convolution of the template spectra
to the instrument’s spectral resolution. To do so, the chosen library
needs to have higher resolution than the observed data. However,
the MUSE spectral resolution is not constant with wavelength, being
lower in the blue part of the spectrum (4750 to 5500 Å) and higher
in the red part (8400 to 9600 Å) (Gue´rou et al. 2015; Krajnovic´ et al.
2015; Mentz et al. 2016; Vaughan et al. 2018; Emsellem et al. 2019).
The instrument resolution in the red part of the spectra is even higher
than the spectral resolution of the E-MILES library. This makes the
measurement of the velocity dispersion in low-surface-brightness
galaxies, such as dEs, challenging as the velocity dispersion of the
targeted dEs can be lower than the instrument resolution. This issue
has already been mentioned in different studies of low-mass systems
(e.g. see Emsellem et al. 2004, 2019; Ganda et al. 2006; Johnston
et al. 2019)
Following Emsellem et al. (2019) and in order to test the robustness
of our velocity dispersion measurements and their dependence on
the library resolution, we repeated the fitting procedure by using the
calcium II triplet (CaT) library (Cenarro et al. 2001), encompassing
the wavelength range of the Ca triplet lines, which has a higher
resolution than E-MILES. The CaT library contains 706 stellar
spectra covering the [Fe/H] range from −3.45 to 0.6 dex. This library
is limited to the spectral range of 8350–9020 Å with a FWHM ∼
1.5 Å (Cenarro et al. 2001). Due to the small spectral coverage
of the CaT library, we performed the PPXF fitting between 8400
and 9000 Å. Adopting an instrument FWHM of 2.51 Å we present
the results of this test in Fig. B1 for three dEs. In all panels, the
distribution of the velocity dispersion as measured from each bin’s
average spectrum using E-MILES and the full spectrum fitting option
is traced in blue, while the distribution of velocity dispersion derived
with the CaT library is plotted in red. The mean value of each
distribution is shown with a thick dashed line while the dotted
lines of the same colour trace the ± 1 standard deviation of the
distribution.
As shown in Fig. B1, and consistent with the results of Emsellem
et al. (2019) albeit for different sets of stellar libraries, fitting using
both the E-MILES and CaT libraries retrieves a similar range of
values for the velocity dispersion in three dEs of our sample. This
means that the velocity dispersion of the stellar component obtained
in this study from the full spectrum fitting is reliable and is not
particularly affected by the instrument or library resolution. This
is true for six dEs in our sample. The remaining three dEs have
Figure B1. The distribution of velocity dispersion measured with (1) the full
spectrum fitting using the E-MILES library (in blue) and (2) the Ca triplet
lines using the CaT library (in red) for three dEs in our sample. The mean
value of each distribution is shown with a thick dashed line, while the thin
dotted lines represent the standard deviation of the mean of the distribution.
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Dwarf kinematics in a group accreted to Virgo 1919
lower SNR due to shorter exposure time (VCC1833) or extended size
(VCC 1836 and 0794) that cannot be fully mapped using MUSE. For
these reasons, the sky background was not properly modelled and
subtracted, thus hampering the quality of the fits in the CaT region
(for the effect of sky residuals also check Johnston et al. 2019). As an
additional test, we used the varying resolution that mimics the MUSE
instrument resolution, introduced by Gue´rou et al. (2015) and Bacon
et al. (2017). Results of this test showed no particular difference in
comparison to those obtained by using a fixed resolution of FWHM
= 2.51 Å (Bacon et al. 2010)
APPENDI X C : ERROR MAPS
In Fig. C1, we provide error maps of velocity and velocity dispersion.
Errors are measured through Monte Carlo simulations, as described
in Section 4.
Figure C1. Error maps of velocity and velocity dispersion for our sample of dEs. For a better comparison, the isophotes from Figs 3 and 4 are repeated in all
the panels here.
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Figure C1 – continued
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Dwarf kinematics in a group accreted to Virgo 1921
Figure C1 – continued
APPEN D IX D : EMISSION LINES IN THE
C E N T R E O F V C C 0 1 7 0
As mentioned in Section 4.2, nebular emission lines were detected
in the centre of VCC0170. We summed all the MUSE spaxels within
the central region of this galaxy (as shown in Fig. 5), modelled the
emission of the underlying stellar population with PPXF, and finally
subtracted this model to obtain the nebular optical emission lines of
H β, [O III] λ 4959, [O III] λ 5007, [N II] λ 6548, H α, [N II] λ 6583,
[S II] λ 6717, and [S II] λ 6731. These emission lines are shown in
Fig. D1. We corrected the H β and H α fluxes for Galactic foreground
extinction using Av = 0.089 mag from Schlafly & Finkbeiner (2011).
We also used the following Balmer decrement to correct the nebular
emission lines for intrinsic reddening:
E(B − V ) = log(f(Hα)/f(Hβ ) × 2.85)) × (0.4(κa − κb))−1, (D1)
where f(Hα) and f(Hβ ) are the observed Hα and Hβ fluxes while
κa and κb are defined based on equations (3a) and (3b) of Cardelli,
Clayton & Mathis (1989). We derived E(B–V) = 0.018 mag. In both
cases, corrections were carried out using extinction law of Cardelli
et al. (1989). We obtained a Hα luminosity of 1.75 × 1038 W. The
observed fluxes of the identified emission lines are listed in Table D1.
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Figure D1. The observed nebular spectrum of the central region in VCC0170, corrected for the underlying galaxy emission. From left to right, the first panel
shows: H β, [O III] λ 4958, and [O III] λ 5007; the second panel: [N II] λ 6548, H α, [N II] λ 6583, and the third shows: [S II] λ 6717 and [S II] λ 6731.
Table D1. List of optical emission lines and their ob-
served fluxes in VCC0170.
Emission line Observed flux
×10−15(erg s−1 cm−2)
H α 4.76
H β 1.64
[O III] λ4959 0.20
[O III] λ5007 0.66
[N II] λ6548 0.35
[N II] λ6583 1.04
[S II] λ6717 1.12
[S II] λ6731 0.80
Note. Columns are: Name of nebular emission line,
observed flux.
A PPENDIX E: N EWLY DISCOV ERED
GALAXIES IN THE MUSE CUBES
As mentioned in Section 4, nebular emission lines were detected in a
particular region inside VCC 1836 in the vicinity of the dE’s centre.
Apart from the bright nucleus of VCC1836, the SDSS images of this
dE also show a faint light concentration in the central region of this
galaxy. Thanks to the high-spatial resolution of MUSE, this detected
source is better resolved in the MUSE stacked images of our data
set, as shown in Fig. E1 (red box). We extracted the spectrum of this
particular region (hereafter BB1) within an aperture with a radius
of 5 pixels, corresponding to an area of 3 arcsec2. We subtracted
the possible light contamination of VCC 1836 by subtracting a
spectrum extracted within an annulus of the same size around the
detected source. The final spectrum is plotted in the right-hand panel
of Fig. E1. We measured the redshift of this background galaxy,
using the Doppler shift of each nebular emission line. Based on the
average redshift, this galaxy is located at z = 0.5528 ± 0.0002 (also
reported in Table E1). We modelled the continuum of this spectrum
using PPXF in order to subtract the emission of the underlying stellar
population and to measure the flux of the nebular emission lines. The
values are reported in the second column of Table E2.
Another source with strong emission lines was detected in the
MUSE cube of VCC 0170 (hereafter BB2). In Fig. E2 we marked
the location of this extended source with a red box. As in the previous
case, we extracted the spectrum of this source within a radius of 7
pixels, corresponding to an area of 6 arcsec2. The sky and the VCC
0170 emission were removed by subtracting a spectrum extracted
within an aperture of the same size near the source. The resulting
spectrum is plotted in the right-hand panel of Fig. E2. We measured
the redshift of this source using the nebular emission lines. This
background galaxy is at a redshift z = 0.349 ± 0.013. We modelled
the continuum and absorption features using PPXF and subtracted
the model from the source spectrum to measure the line fluxes. They
are reported in the third column of Table E2.
To locate this galaxy in the BPT diagram (Baldwin et al. 1981), we
corrected the emission lines of H α, [O III] λ 5007, H β and [N II] for
Galactic foreground extinction, using Av = 0.089 mag as reported
by Schlafly & Finkbeiner (2011). The same emission lines were
also corrected for intrinsic reddening using the Balmer decrement as
described in Section D (E(B–V) = 0.055 mag). For both corrections
we used the extinction law of Cardelli et al. (1989). We also corrected
the observed flux for redshift dimming following Calvi et al. (2014):
I = I0(1 + z)−4, (E1)
where I and I0 are the observed and intrinsic surface brightness.
Following the corrections above, we measured log[[O III]λ5007/H β]
= −0.038 and log[[N II] λ6584/H α] = −0.351. According to these
values, the detected galaxy is located at the edge of the star forming
galaxies in the BPT diagram, as defined by the line of Kauffmann
et al. (2003). We used the measured redshift to estimate the comoving
radial distance, which turns out to be ≈1360.4 Mpc. Using this value,
we measured the star formation rate of this background galaxy to be
0.66 M yr−1, following the method of Calzetti et al. (2007).
Further analysis of the gas emission lines present in the spectrum
of this galaxy shows that the H α, [S II], and [N II] lines are split
(Fig. E3). This can likely be due to the rotation of this background
galaxy. We used the split in the lines and measured the rotation
of this galaxy to be of the order of ≈90 km s−1, with an error of
approximately 10 km s−1.
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Dwarf kinematics in a group accreted to Virgo 1923
Figure E1. A source detected in the projected vicinity of the central region of VCC1836. Left-hand panel: The location of the detected source (BB1) is marked
with a red box with a size of 8 × 8 arcsec2 on the MUSE stacked image. Right-hand panel: The observed spectrum of the detected source, corrected for the
underlying emission of VCC1836. The following emission lines are marked from left to right: O[ II] λ 3727, H γ , H β, [O III] λ 4958, [O III] λ 5007.
Table E1. New galaxies in the MUSE cubes: BB1 and BB2.
Name RA (J2000) Dec (J2000) redshift total MV, AB
BB1 12 40 19.50 +14 43 0.0 0.5528 ± 0.0002 21.32
BB2 12 14 20.50 +14 26 2.0 0.349 ± 0.013 21.50
Note. Columns are: Name of the target, RA, DEC, redshift and total AB
magnitude in V band.
Table E2. List of optical emission lines and their observed fluxes in BB1
and BB2.
Emission Observed Flux in BB1 Observed Flux in BB2
Lines ×10−17 (erg s−1 cm−2) ×10−17 (erg s−1 cm−2)
H α – 15.8
H γ 3.62 –
H β 7.43 51.5
[O II] 14.7 17.6
[O III] λ 4959 1.68 1.66
[O III] λ 5007 5.64 4.86
[N II] λ 6548 0.35 2.33
[N II] λ 6583 1.04 7.08
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Figure E2. Detected background source in the FOV of VCC0170. Left-hand panel: The location of the detected source (BB2) is marked with a red box with a
size of 8 × 8 arcsec2 on the MUSE stacked image. Right-hand panel: The observed spectrum of the detected source, corrected for the underlying emission of
VCC0170. Marked emission lines from left to right: [O II] λ 3727, H β, [O III] λ 4958, [O III] λ 5007, H α, [N II] λ 6548, [N II] λ 6583, [S II] λ 6717, and [S II] λ
6731.
Figure E3. Split H α and [N II] 6583 emission lines in the red part of the spectrum of BB2. This split corresponds to a rotation velocity of 90 ± 10 km s−1.
This paper has been typeset from a TEX/LATEX file prepared by the author.
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