93 research outputs found
The puzzling nature of dwarf-sized gas poor disk galaxies
Early-type dwarf galaxies were originally described as elliptical feature-less galaxies. However, later disk signatures were revealed in some of them. In fact, it is still disputed whether they follow photometric scaling relations similar to giant elliptical galaxies or whether they are rather formed in transformations of late-type galaxies induced by the galaxy cluster environment. The early-type dwarf galaxies are the most abundant galaxy type in clusters, and their low-mass make them susceptible to processes that let galaxies evolve. Therefore, they are well-suited as probes of galaxy evolution.
In this thesis we explore possible relationships and evolutionary links of early-type dwarfs to other galaxy types. We observed a sample of 121 galaxies and obtained deep near-infrared images. For analyzing the morphology of these galaxies, we apply two-dimensional multicomponent fitting to the data. This is done for the first time for a large sample of early-type dwarfs. A large fraction of the galaxies is shown to have complex multicomponent structures.
The photometric parameters of the inner and outer components of the dwarfs are compared to bulge and disk components of other galaxy types from studies using a similar decomposition approach. The parameters of the bulges and disks form rather tight relations of size as a function of galaxy brightness. The inner and outer components of the dwarfs are offset from the extrapolations of these scaling relations, and we conclude that their nature is different. Complementary N-body simulations illustrate that the inner and outer components of the dwarfs may indeed have been formed from the disks of late-type galaxies that are ha- rassed by the cluster environment. The multiple structure components may also explain our finding that the early-type dwarf galaxies show subtle deviations from common scaling relations with bright elliptical galaxies, i.e. the relation of size versus galaxy brightness. The dwarfs and giants would be expected to follow jointly one such relation, based on the observed, continuos variation of the light profiles, if all the galaxies followed simple one-component profiles.
Altogether our results indicate that many of the early-type dwarf galaxies may be disk galaxies. This view is supported by the fact that their sizes and galaxy brightnesses place the early-type dwarfs close to the extrapolation of the scaling relation of these parameters of disks in bright galaxies, but offset from that of the bulges. A possible explanation for early-type dwarfs beings disks is that they have been transformed from late-type disk galaxies by the cluster environment.
Furthermore, we demonstrate that processes that may be responsible for such transformations are also viable options for: (i) explaining that the slowly moving nucleated early-type dwarfs in the cluster center are round, while the fast moving are flat, (ii) making blue-compact dwarfs to evolve into early-type dwarfs, and (iii) forming ultra-compact dwarfs by disruption of nucleated early-type dwarfs
Signatures of quenching in dwarf galaxies in local galaxy clusters
The transformation of late-type galaxies has been suggested as the origin of
early-type dwarf galaxies in galaxy clusters. Venhola et al. analysed
correlations between colour and surface brightness for galaxies in the Fornax
cluster binned by luminosity or stellar mass. In the bins with , the authors identified a correlation of redness with fainter
surface brightness and interpreted it as a consequence of the quenching of star
formation by ram pressure stripping in the dwarf galaxies. We carry out a
corresponding analysis for the Virgo cluster and find great similarities in
these correlations between surface brightness and colour for the two clusters,
despite expected differences in the strength of the ram pressure. Furthermore,
we extend the analysis to a wider range of optical colours for both clusters
and contrast the results with expectations for fading and reddening stellar
populations. Overall the slopes of the surface brightness-colour relations are
consistent with these models. In addition the sizes of the early- and late-type
galaxies at these low masses are comparable. These two results are compatible
with a transformation scenario. However, when analysing early- and late-type
galaxies separately, the consistency of the slope of the surface
brightness-colour relations with the model expectations for fading and
reddening stellar population applies only to the late types. The lack of this
imprint for the early-type dwarfs calls for some additional explanation, for
which we discuss several possibilities. Finally, the Virgo cluster is an
atypical cluster with a low fraction of quiescent early-type galaxies at all
galaxy masses despite its large cluster mass. (abridged)Comment: 17 pages, 16 figures, accepted for publication in A&
Implications for the origin of dwarf early-type galaxies: a detailed look at the isolated rotating dwarf early-type galaxy CG 611, with ramifications for the Fundamental Plane's (S_K)^2 kinematic scaling and the spin-ellipticity diagram
Selected from a sample of nine, isolated, dwarf early-type galaxies (ETGs)
having the same range of kinematic properties as dwarf ETGs in clusters, we use
CG 611 (LEDA 2108986) to address the Nature versus Nurture debate regarding the
formation of dwarf ETGs. The presence of faint disk structures and rotation
within some cluster dwarf ETGs has often been heralded as evidence that they
were once late-type spiral or dwarf irregular galaxies prior to experiencing a
cluster-induced transformation into an ETG. However, CG 611 also contains
significant stellar rotation (~20 km/s) over its inner half light radius,
R_(e,maj)=0.71 kpc, and its stellar structure and kinematics resemble those of
cluster ETGs. In addition to hosting a faint young nuclear spiral within a
possible intermediate-scale stellar disk, CG 611 has accreted an
intermediate-scale, counter-rotating gas disk. It is therefore apparent that
dwarf ETGs can be built by accretion events, as opposed to disk-stripping
scenarios. We go on to discuss how both dwarf and ordinary ETGs with
intermediate-scale disks, whether under (de)construction or not, are not fully
represented by the kinematic scaling S_0.5=sqrt{ 0.5(V_rot)^2 + sigma^2 }, and
we also introduce a modified spin-ellipticity diagram, lambda(R)-epsilon(R),
with the potential to track galaxies with such disks.Comment: 15 pages (includes 9 figures and an extensive 2+ page reference list
On the Color Magnitude Relation of Early-type Galaxies
In this letter we present a study of the color magnitude relation of 468
early-type galaxies in the Virgo Cluster with Sloan Digital Sky Survey imaging
data. The analysis of our homogeneous, model-independent data set reveals that,
in all colors (u-g, g-r, g-i, i-z) similarly, giant and dwarf early-type
galaxies follow a continuous color magnitude relation (CMR) that is best
described by an S-shape. The magnitude range and quality of our data allows us
to clearly confirm that the CMR in Virgo is not linear. Additionally, we
analyze the scatter about the CMR and find that it increases in the
intermediate-luminosity regime. Nevertheless, despite this observational
distinction, we conclude from the similarly shaped CMR of semi-analytic model
predictions that dwarfs and giants could be of the same origin.Comment: Accepted by ApJL; 5 pages, 3 figures; added missing line to Figure
LEDA 074886: A remarkable rectangular-looking galaxy
We report the discovery of an interesting and rare, rectangular-shaped
galaxy. At a distance of 21 Mpc, the dwarf galaxy LEDA 074886 has an absolute
R-band magnitude of -17.3 mag. Adding to this galaxy's intrigue is the presence
of an embedded, edge-on stellar disk (of extent 2R_{e,disk} = 12 arcsec = 1.2
kpc) for which Forbes et al. reported V_rot/sigma ~ 1.4. We speculate that this
galaxy may be the remnant of two (nearly edge-one) merged disk galaxies in
which the initial gas was driven inward and subsequently formed the inner disk,
while the stars at larger radii effectively experienced a dissipationless
merger event resulting in this `emerald cut galaxy' having very boxy isophotes
with a_4/a = -0.05 to -0.08 from 3 to 5 kpc. This galaxy suggests that
knowledge from simulations of both `wet' and `dry' galaxy mergers may need to
be combined to properly understand the various paths that galaxy evolution can
take, with a particular relevance to blue elliptical galaxies.Comment: To appear in ApJ. Six pages including references and figure
Signatures of quenching in dwarf galaxies in local galaxy clusters: A comparison of the galaxy populations in the Virgo and Fornax clusters
Context. The transformation of late-type galaxies has been suggested as the origin of early-type dwarf galaxies (typically Mâââ€â109âMâ) in galaxy clusters. Based on deep images, Venhola and colleagues analysed correlations between colour and surface brightness for galaxies in the Fornax cluster binned by luminosity or stellar mass. In the bins with Mââ8âMâ, the authors identified a correlation of redness with fainter surface brightness and interpreted it as a consequence of the quenching of star formation by ram pressure stripping in the dwarf galaxies.Aims. This study carries out a similar analysis for the Virgo cluster. The analysis for both clusters is then used to compare the Virgo and Fornax clusters, for which the ram pressure is expected to have different strengths. The purpose of this is to scrutinise the ram pressure interpretation from the other study and search for differences between the clusters that reflect the different ram pressure efficiencies, which would either support or weaken this interpretation. Ultimately, this could help weigh the importance of ram pressure stripping relative to other transformative processes in the shaping of the dominant early-type dwarf galaxy population.Methods. We extend the analysis of colour versus surface brightness binned by stellar mass to higher masses and a wider range of optical colours. The results, in particular at low stellar mass, are compared to predictions of stellar evolution models. Benefitting from larger sample sizes, we also analyse late- and early-type galaxies separately. This analysis is carried out for the Virgo and Fornax clusters, and the colour versus surface brightness relation, as well as other properties of the two clustersâ galaxy populations, are compared.Results. While the colourâsurface brightness diagrams are remarkably similar for the two clusters, only the low-mass late-type galaxies are found to have slopes consistent with a fading and reddening following the quenching of star formation. For the early-type galaxies, there are no (or only weak) correlations between colour and surface brightness in all mass bins. Early- and late-type galaxies in both clusters have comparable sizes below a stellar mass of MâââČâ108âMâ. The colour and size scaling relations are very similar for the Virgo and Fornax clusters. However, Virgo features a lower fraction of early-type or red galaxies despite its higher mass.Conclusions. The similarity of early-type dwarfs and low-mass late types in size at the masses MâââČâ108âMâ as well as the overall consistency of the colourâsurface brightness correlation with fading stellar populations support a scenario of transformation via the quenching of star formation, for example by gas removal. However, the lack of this imprint of an ageing stellar population on the early-type dwarfs themselves calls for some additional explanation. Finally, the Virgo cluster is an atypical cluster with a comparably low fraction of quiescent early-type galaxies at all galaxy masses despite its large cluster mass.Key words: galaxies: clusters: general / galaxies: evolution / galaxies: dwarf / galaxies: stellar content / galaxies: clusters: individual: Virgo / galaxies: clusters: individual: Fornax© ESO 2021</p
Young, metal-enriched cores in early-type dwarf galaxies in the Virgo cluster based on colour gradients
Early-type dwarf galaxies are not simply featureless, old objects, but were
found to be much more diverse, hosting substructures and a variety of stellar
population properties. To explore the stellar content of faint early-type
galaxies, and to investigate in particular those with recent central star
formation, we study colours and colour gradients within one effective radius in
optical (g-r) and near-infrared (i-H) bands for 120 Virgo cluster early types
with -19 mag < < -16 mag. Twelve galaxies turn out to have blue cores,
when defined as g-r colour gradients larger than 0.10 mag/, which
represents the positive tail of the gradient distribution. For these galaxies,
we find that they have the strongest age gradients, and that even outside the
blue core, their mean stellar population is younger than the mean of ordinary
faint early types. The metallicity gradients of these blue-cored early-type
dwarf galaxies are, however, in the range of most normal faint early types,
which we find to have non-zero gradients with higher central metallicity. The
blue central regions are consistent with star formation activity within the
last few 100 Myr. We discuss that these galaxies could be explained by
environmental quenching of star formation in the outer galaxy regions, while
the inner star formation activity has continued
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