126 research outputs found
Faint dwarf galaxies in nearby clusters
Besides giant elliptical galaxies, a number of low-mass stellar systems
inhabit the cores of galaxy clusters, such as dwarf elliptical galaxies
(dEs/dSphs), ultra-compact dwarf galaxies (UCDs), and globular clusters. The
detailed morphological examination of faint dwarf galaxies has, until recently,
been limited to the Local Group (LG) and the two very nearby galaxy clusters
Virgo and Fornax. Here, we compare the structural parameters of a large number
of dEs/dSphs in the more distant clusters Hydra I and Centaurus to other
dynamically hot stellar systems.Comment: 2 pages, 1 figure; to appear in "A Universe of Dwarf Galaxies:
Observations, Theories, Simulations", held in Lyon, France (June 14-18,
2010), eds. M. Koleva, P. Prugniel & I. Vauglin, EAS Series (Paris: EDP
The specific frequencies of ultra-compact dwarf galaxies
We aim at quantifying the specific frequency of UCDs in a range of
environments and at relating this to the frequency of globular clusters (GCs)
and potential progenitor dwarf galaxies. Are the frequencies of UCDs consistent
with being the bright tail of the GC luminosity function (GCLF)? We propose a
definition for the specific frequency of UCDs,
S_{N,UCD}=N_{UCD}*10^{0.4*(M_{V,host}-M_{V,0})}*c_{w}. The parameter M_{V,0} is
the zeropoint of the definition, chosen such that the specific frequency of
UCDs is the same as those of globular clusters, S_{N,GC}, if UCDs follow a
simple extrapolation of the GCLF. The parameter c_{w} is a correction term for
the GCLF width sigma. We apply our definition of S_{N,UCD} to results of
spectroscopic UCD searches in the Fornax, Hydra and Centaurus galaxy clusters,
two Hickson Compact Groups, and the Local Group. This includes a large database
of 180 confirmed UCDs in Fornax. We find that the specific frequencies derived
for UCDs match those of GCs very well, to within 10-50%. The ratio
{S_{N,UCD}}/{S_{N,GC}} is 1.00 +- 0.44 for the four environments Fornax, Hydra,
Centaurus, and Local Group, which have S_{N,GC} values. This good match also
holds for individual giant galaxies in Fornax and in the Fornax
intracluster-space. The error ranges of the derived UCD specific frequencies in
the various environments then imply that not more than 50% of UCDs were formed
from dwarf galaxies. We show that such a scenario would require >90% of
primordial dwarfs in galaxy cluster centers (<100 kpc) to have been stripped of
their stars. We conclude that the number counts of UCDs are fully consistent
with them being the bright tail of the GC population. From a statistical point
of view there is no need to invoke an additional formation channel.Comment: 11 pages, 6 figures, A&A accepted. Press release
http://www.aanda.org/index.php?option=com_content&task=view&id=788&Itemid=27
The dark halo of the Hydra I galaxy cluster: core, cusp, cosmological? Dynamics of NGC 3311 and its globular cluster system
NGC 3311 is the central cD galaxy of the Hydra I cluster. We use globular
clusters around NGC 3311, combined with kinematical data of the galaxy itself,
to investigate the dark matter distribution in the central region of Hydra I.
Radial velocities of 118 bright globular clusters, based on VLT/VIMOS mask
spectroscopy, are used to calculate velocity dispersions which are well defined
out to 100 kpc. NGC 3311 is the most distant galaxy for which this kind of
study has been performed. We also determine velocity dispersions of the stellar
component from long slit spectroscopy out to 20 kpc. Moreover, we present a new
photometric model for NGC 3311 in the V-band. We search for a dark halo which
in the context of a spherical Jeans model. We also compare the radial velocity
distributions of globular clusters and planetary nebulae. The projected stellar
velocity dispersion rises from 185 km/s to 350 km/s at a radius of 20 kpc. The
globular cluster dispersion rises as well from 500 km/s at 10 kpc to about 800
km/s at 100 kpc, comparable to the velocity dispersion of the cluster galaxies.
A dark matter halo with a core reproduces well the velocity dispersions of
stars and globular clusters simultaneously under isotropy. The central stellar
velocity dispersions predicted by cosmological NFW halos are less good
representations, while the globular clusters allow a wide range of halo
parameters. A suspected radial anisotropy of the stellar population aggravates
the deviations. However, we find discrepancies with previous kinematical data,
which we cannot resolve and may indicate a more complicated velocity pattern.
Although one cannot conclusively demonstrate that the dark matter halo of NGC
3311 has a core rather than a cusp, a core seems to be preferred by the present
data. A more complete velocity field and an analysis of the anisotropy is
required to reach firm conclusions.Comment: 8 pages, 5 figures, abstract abridged, accepted for publication in
A&
A search for massive UCDs in the Centaurus Galaxy Cluster
We recently initiated a search for ultra-compact dwarf galaxies (UCDs) in the
Centaurus galaxy cluster (Mieske et al. 2007), resulting in the discovery of 27
compact objects with -12.2<M_V<-10.9 mag. Our overall survey completeness was
15-20% within 120 kpc projected clustercentric distance. In order to better
constrain the luminosity distribution of the brightest UCDs in Centaurus, we
continue our search by substantially improving our survey completeness
specifically in the regime M_V<-12 mag (V_0<21.3 mag). Using VIMOS at the VLT,
we obtain low-resolution spectra of 400 compact objects with 19.3<V_0<21.3 mag
(-14<M_V<-12 mag at the Centaurus distance) in the central 25' of the Centaurus
cluster, which corresponds to a projected radius of ~150 kpc. Our survey yields
complete area coverage within ~120 kpc. For 94% of the sources included in the
masks we successfully measure a redshift. Due to incompleteness in the slit
assignment, our final completeness in the area surveyed is 52%. Among our
targets we find three new UCDs in the magnitude range -12.2<M_V<-12 mag, hence
at the faint limit of our survey. One of them is covered by archival HST WFPC2
imaging, yielding a size estimate of r_h <= 8-9 pc. At 95% confidence we can
reject the hypothesis that in the area surveyed there are more than 2 massive
UCDs with M_V<-12.2 mag and r_eff <=70 pc. Our survey hence confirms the
extreme rareness of massive UCDs. We find that the radial distributions of
Centaurus and Fornax UCDs with respect to their host clusters' centers agree
within the 2 sigma level.Comment: 9 pages, 7 figures, accepted as Research Note for A&
Families of dynamically hot stellar systems over ten orders of magnitude in mass
Dynamically hot stellar systems, whether star clusters or early-type
galaxies, follow well-defined scaling relations over many orders of magnitudes
in mass. These fundamental plane relations have been subject of several
studies, which have been mostly confined to certain types of galaxies and/or
star clusters so far. Here, we present a complete picture of hot stellar
systems ranging from faint galaxies and star clusters of only a few hundred
solar masses up to giant ellipticals (gEs) with 10^12 M_sun, in particular
including large samples of compact ellipticals (cEs), ultra-compact dwarf
galaxies (UCDs), dwarf ellipticals (dEs) of nearby galaxy clusters and Local
Group ultra-faint dwarf spheroidals (dSphs). For all those stellar systems we
show the effective radius-luminosity, effective radius-stellar mass, and
effective mass surface density-stellar mass plane. Two families of hot stellar
systems can be differentiated: the 'galaxian' family, ranging from gEs over Es
and dEs to dSphs, and the 'star cluster' family, comprising globular clusters
(GCs), UCDs and nuclear star clusters (NCs). Interestingly, massive ellipticals
have a similar size-mass relation as cEs, UCDs and NCs, with a clear common
boundary towards minimum sizes. No object of either family is located in the
'zone of avoidance' beyond this limit. Even the majority of early-type galaxies
at high redshift obeys this relation. The sizes of dEs and dSphs as well as GCs
barely vary with mass over several orders of magnitude. We use the constant
galaxy sizes to derive the distances of several local galaxy clusters. Both,
galaxies and star clusters, do not exceed a surface density of \Sigma_eff =
3.17*10^{10}*M^{-3/5} M_sun pc^{-2}, causing an orthogonal kink in the galaxy
sequence for ellipticals more massive than 10^{11} M_sun. The densest stellar
systems (within their effective radius) are nuclear star clusters. (abridged)Comment: 13 pages, 7 figures, accepted for publication in MNRAS. Errors for
mean effective radii and related quantities in Table 3 corrected, references
added and affiliation changed in the replaced versio
A large population of ultra-compact dwarf galaxies in the Hydra I cluster
We performed a large spectroscopic survey of compact, unresolved objects in
the core of the Hydra I galaxy cluster (Abell 1060), with the aim of
identifying ultra-compact dwarf galaxies (UCDs), and investigating the
properties of the globular cluster (GC) system around the central cD galaxy NGC
3311. We obtained VIMOS medium resolution spectra of about 1200 candidate
objects with apparent magnitudes 18.5 < V < 24.0 mag, covering both the bright
end of the GC luminosity function and the luminosity range of all known UCDs.
By means of spectroscopic redshift measurements, we identified 118 cluster
members, from which 52 are brighter than M_V = -11.0 mag, and can therefore be
termed UCDs. The brightest UCD in our sample has an absolute magnitude of M_V =
-13.4 mag (corresponding to a mass of > 5 x 10^7 M_sun) and a half-light radius
of 25 pc. This places it among the brightest and most massive UCDs ever
discovered. Most of the GCs/UCDs are both spatially and dynamically associated
to the central cD galaxy. The overall velocity dispersion of the GCs/UCDs is
comparable to what is found for the cluster galaxies. However, when splitting
the sample into a bright and a faint part, we observe a lower velocity
dispersion for the bright UCDs/GCs than for the fainter objects. At a dividing
magnitude of M_V = -10.75 mag, the dispersions differ by more than 200 km/s,
and up to 300 km/s for objects within 5 arcmin around NGC 3311. We interpret
these results in the context of different UCD formation channels, and conclude
that interaction driven formation seems to play an important role in the centre
of Hydra I.Comment: 18 pages, 17 figures, accepted for publication in A&
The early-type dwarf galaxy population of the Centaurus cluster
We present a photometric study of the early-type dwarf galaxy population of
the Centaurus cluster, aiming at investigating the galaxy luminosity function
(LF) and galaxy scaling relations down to the regime of galaxies with M_V~-10
mag. On deep VLT/FORS1 V- and I-band images of the central part of the cluster,
we identify cluster dwarf-galaxy candidates using both morphological and
surface brightness selection criteria. Photometric and structural parameters of
the candidates are derived from analysis of their surface brightness profiles.
Fundamental scaling relations, such as the colour-magnitude and the
magnitude-surface brightness relation, are used to distinguish the cluster from
the background. We find a flat LF with a slope of \alpha = -1.14 \pm 0.12 for
M_V>-14 mag, when fitting a power law to the completeness-corrected galaxy
number counts. When plotting the central surface brightness of a Sersic model
vs. the galaxy magnitude, we find a continuous relation for magnitudes
-20<M_V<-10 mag, with only the brightest core galaxies deviating from this
relation, in agreement with previous studies of other clusters. In a
size-luminosity diagram of early-type galaxies from a range of environments, we
observe that R_eff slowly decreases with decreasing luminosity for -21<M_V<-13
mag and decreases more rapidly at fainter magnitudes. This trend continues to
the ultra-faint Local Group dwarf galaxies (M_V~-4 mag). The continuous central
surface brightness vs. absolute magnitude relation and the smooth relation in
the size-luminosity diagram over a wide range of magnitudes are consistent with
the interpretation of dwarf galaxies and more massive elliptical galaxies being
one family of objects with gradually changing structural properties. The most
massive core galaxies and the rare cE galaxies are the only exceptions.Comment: 14 pages, 10 figures, 4 tables, accepted for publication in A&
A deep view on the Virgo cluster core
In this study we investigate the optical photometric properties of early-type
galaxies in the Virgo cluster core region, by analysing their location on the
colour magnitude relation (CMR) and the structural scaling relations down to
faint magnitudes, and by constructing the luminosity function to compare it
with theoretical expectations. We visually select potential cluster members
based on morphology and angular size, excluding spiral galaxies. A photometric
analysis has been carried out for 295 galaxies, using surface brightness
profile shape and colour as further criteria to identify probable background
contaminants. 216 galaxies are considered to be certain or probable Virgo
cluster members. Our study reveals 77 galaxies not catalogued in the VCC (with
13 of them already found in previous studies) that are very likely Virgo
cluster members because they follow the Virgo CMR and exhibit low Sersic
indices. Those galaxies reach -8.7 mag in V band. The CMR shows a clear change
in slope from dEs to dSphs, while the scatter of the CMR in the dSph regime
does not increase significantly. Our sample might, however, be somewhat biased
towards redder colours. The scaling relations given by the dEs appear to be
continued by the dSphs indicating a similar origin. The observed change in the
CMR slope may mark the point at which gas loss prevented significant metal
enrichment. The almost constant scatter around the CMR possibly indicates a
short formation period, resulting in similar stellar populations. The
luminosity function shows a Schechter function's faint end slope of
-1.50\pm0.17, implying a lack of galaxies related to the expected number of
low-mass dark matter haloes from theoretical models. Our findings could be
explained by suppressed star formation in low-mass dark matter halos or by
tidal disruption of dwarfs in the dense core region of the cluster.Comment: A&A accepted, photometry of newly identified dwarf galaxies is
provided in Table 3, 17 pages, 8 figure
Two adhesive systems cooperatively regulate axon ensheathment and myelin growth in the CNS
Central nervous system myelin is a multilayered membrane produced by oligodendrocytes to increase neural processing speed and efficiency, but the molecular mechanisms underlying axonal selection and myelin wrapping are unknown. Here, using combined morphological and molecular analyses in mice and zebrafish, we show that adhesion molecules of the paranodal and the internodal segment work synergistically using overlapping functions to regulate axonal interaction and myelin wrapping. In the absence of these adhesive systems, axonal recognition by myelin is impaired with myelin growing on top of previously myelinated fibers, around neuronal cell bodies and above nodes of Ranvier. In addition, myelin wrapping is disturbed with the leading edge moving away from the axon and in between previously formed layers. These data show how two adhesive systems function together to guide axonal ensheathment and myelin wrapping, and provide a mechanistic understanding of how the spatial organization of myelin is achieved
Two formation channels of UCDs in Hickson Compact Groups
The formation of ultra-compact dwarf galaxies (UCDs) is believed to be
interaction driven, and UCDs are abundant in the cores of galaxy clusters,
environments that mark the end-point of galaxy evolution. Nothing is known
about the properties of UCDs in compact groups of galaxies, environments where
most of galaxy evolution and interaction is believed to occur and where UCDs in
intermediate state of evolution may be expected. The main goal of this study is
to detect and characterize, for the first time, the UCD population of compact
groups. For that, 2 groups in different evolutionary stages, HCG 22 and HCG 90,
were targeted with VLT/FORS2/MXU. We detect 16 and 5 objects belonging to HCG
22 and HCG 90, respectively, covering the magnitude range -10.0 > M_R > -11.5
mag. Their colours are consistent with old ages covering a broad range in
metallicities. Photometric mass estimates put 4 objects in HCG 90 and 9 in HCG
22 in the mass range of UCDs (>2x10^6 M_Sun) for an assumed age of 12 Gyr.
These UCDs are on average 2-3 times larger than typical Galactic GCs, covering
a range of 2 >~ r_h >~ 21 pc. The UCDs in HCG 22 are more concentrated around
the central galaxy than in HCG 90, at the 99% confidence level. They cover a
broad range in [alpha/Fe] abundances from sub- to super-solar. The spectra of 3
UCDs show tentative evidence for intermediate age stellar populations. We
calculate the specific frequency (S_N) of UCDs for both groups, finding that
HCG 22 has about three times higher S_N than HCG 90. The ensemble properties of
the detected UCDs supports 2 co-existing formation channels: a star cluster
origin and an origin as tidally stripped dwarf nuclei. Our results imply that
the UCDs detected in both groups do not, in their majority, originate from
relatively recent galaxy interactions. Most of the detected UCDs have likely
been brought into the group with their host galaxies.[abridged]Comment: Accepted for publication at A&A, 17 pages, 9 figures + 2 additional
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