48 research outputs found
Spatially Resolved Kinematics of an Ultra-Compact Dwarf Galaxy
We present the internal kinematics of UCD3, the brightest known ultra-compact
dwarf galaxy (UCD) in the Fornax cluster, making this the first UCD with
spatially resolved spectroscopy. Our study is based on seeing-limited
observations obtained with the ARGUS Integral Field Unit of the VLT/FLAMES
spectrograph under excellent seeing conditions (0.5 - 0.67 arcsec FWHM). The
velocity field of UCD3 shows the signature of weak rotation, comparable to that
found in massive globular clusters. Its velocity dispersion profile is fully
consistent with an isotropic velocity distribution and the assumption that mass
follows the light distribution obtained from Hubble Space Telescope imaging. In
particular, there is no evidence for the presence of an extended dark matter
halo contributing a significant (>~33 per cent within R < 200 pc) mass
fraction, nor for a central black hole more massive than ~5 per cent of the
UCD's mass. While this result does not exclude a galaxian origin for UCD3, we
conclude that its internal kinematics are fully consistent with it being a
massive star cluster.Comment: 5 pages, 3 figures; accepted for publication in MNRAS Letter
The Nature of UCDs: Internal Dynamics from an Expanded Sample and Homogeneous Database
We have obtained high-resolution spectra of 23 ultra-compact dwarf galaxies
(UCDs) in the Fornax cluster with -10.4>M_V>-13.5 mag (10^6<M/M_*<10^8), using
FLAMES/Giraffe at the VLT. This is the largest homogeneous data set of UCD
internal dynamics assembled to date. We derive dynamical M/L ratios for 15 UCDs
covered by HST imaging. In the M_V-sigma plane, UCDs with M_V<-12 mag are
consistent with the extrapolated Faber-Jackson relation for luminous
ellipticals, while fainter UCDs are closer to the extrapolated globular cluster
(GC) relation. At a given metallicity, Fornax UCDs have on average 30-40% lower
M/L ratios than Virgo UCDs, suggesting possible differences in age or dark
matter content between Fornax and Virgo UCDs. For our sample of Fornax UCDs we
find no significant correlation between M/L ratio and mass. We combine our data
with available M/L ratio measurements of compact stellar systems with
10^4<M/M_*<10^8, and normalise all M/L estimates to solar metallicity. We find
that UCDs (M > 2*10^6 M_*) have M/L ratios twice as large as GCs (M < 2*10^6
M_*). We show that stellar population models tend to under-predict dynamical
M/L ratios of UCDs and over-predict those of GCs. Considering the scaling
relations of stellar spheroids, UCDs align well along the 'Fundamental
Manifold', constituting the small-scale end of the galaxy sequence. The
alignment for UCDs is especially clear for r_e >~ 7 pc, which corresponds to
dynamical relaxation times that exceed a Hubble time. In contrast, GCs exhibit
a broader scatter and do not appear to align along the manifold. We argue that
UCDs are the smallest dynamically un-relaxed stellar systems, with M > 2*10^6
M_* and 7<r_e<100 pc. Future studies should aim at explaining the elevated M/L
ratios of UCDs and the environmental dependence of their properties.Comment: 17 pages, 14 figures, accepted for publication in A&A. V3 taking into
account proof corrections: Table 3 radial velocity entries corrected by
heliocentric correction, updated sigma entries in Table 5 for a few CenA
sources, updated references for G1 and omega Ce
The photometric evolution of dissolving star clusters: II. Realistic models. Colours and M/L ratios
Evolutionary synthesis models are the prime method to construct models of
stellar populations, and to derive physical parameters from observations. One
of the assumptions for such models so far has been the time-independence of the
stellar mass function. However, dynamical simulations of star clusters in tidal
fields have shown the mass function to change due to the preferential removal
of low-mass stars from clusters. Here we combine the results from dynamical
simulations of star clusters in tidal fields with our evolutionary synthesis
code GALEV to extend the models by a new dimension: the total cluster
disruption time. We reanalyse the mass function evolution found in N-body
simulations of star clusters in tidal fields, parametrise it as a function of
age and total cluster disruption time and use this parametrisation to compute
GALEV models as a function of age, metallicity and the total cluster disruption
time. We study the impact of cluster dissolution on the colour (generally, they
become redder) and magnitude (they become fainter) evolution of star clusters,
their mass-to-light ratios (off by a factor of ~2 -- 4 from standard
predictions), and quantify the effect on the cluster age determination from
integrated photometry (in most cases, clusters appear to be older than they
are, between 20 and 200%). By comparing our model results with observed M/L
ratios for old compact objects in the mass range 10^4.5 -- 10^8 Msun, we find a
strong discrepancy for objects more massive than 10^7 Msun (higher M/L). This
could be either caused by differences in the underlying stellar mass function
or be an indication for the presence of dark matter in these objects. Less
massive objects are well represented by the models. The models for a range of
total cluster disruption times are available online. (shortened)Comment: MNRAS, in press, data are available at
http://www.phys.uu.nl/~anders/data/SSP_varMF/, http://data.galev.org and soon
also from CDS ... sorry for the sometimes strange layout, that's LaTe
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
figure
Extranuclear Halpha-emitting complexes in low-z (U)LIRGs: Precursors of tidal dwarf galaxies?
(Abridged)This paper characterizes the physical and kinematic properties of
external massive star-forming regions in a sample of (U)LIRGs. We use high
angular resolution ACS images from the HST B and I bands, as well as
Halpha-line emission maps obtained with IFS. We find 31 external
Halpha-emitting (young star-forming) complexes in 11 (U)LIRGs. These complexes
have in general similar sizes, luminosities, and metallicities to extragalactic
giant HII regions and TDG candidates found in less luminous mergers and compact
groups of galaxies. We assess the mass content and the likelihood of survival
as TDGs of the 22 complexes with simple structures in the HST images based on
their photometric, structural, and kinematic properties. The dynamical tracers
used (radius-sigma and luminosity-sigma diagrams) indicate that most of the
complexes might be self-gravitating entities. The resistance to forces from the
parent galaxy is studied by considering the tidal mass of the candidate and its
relative velocity with respect to the parent galaxy. After combining the
results of previous studies of TDG searches in ULIRGs a total of 9 complexes
satisfy most of the applied criteria and thus show a high-medium or high
likelihood of survival, their total mass likely being compatible with that of
dwarf galaxies. They are defined as TDG candidates. We propose that they
probably formed more often during the early phases of the interaction.
Combining all data for complexes with IFS data where a significant fraction of
the system is covered, we infer a TDG production rate of 0.3 candidates with
the highest probabilities of survival per system for the (U)LIRGs class. This
rate, though, might decrease to 0.1 after the systems in (U)LIRGs have evolved
for 10 Gyr, for long-lived TDGs, which would imply that no more than 5-10 % of
the overall dwarf population could be of tidal origin.Comment: Accepted for publication in A&A, 21 pages, 8 figures. Typo corrected
(article 1111.0468
The stellar and sub-stellar IMF of simple and composite populations
The current knowledge on the stellar IMF is documented. It appears to become
top-heavy when the star-formation rate density surpasses about 0.1Msun/(yr
pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing
metallicity and in increasingly massive early-type galaxies. It declines quite
steeply below about 0.07Msun with brown dwarfs (BDs) and very low mass stars
having their own IMF. The most massive star of mass mmax formed in an embedded
cluster with stellar mass Mecl correlates strongly with Mecl being a result of
gravitation-driven but resource-limited growth and fragmentation induced
starvation. There is no convincing evidence whatsoever that massive stars do
form in isolation. Various methods of discretising a stellar population are
introduced: optimal sampling leads to a mass distribution that perfectly
represents the exact form of the desired IMF and the mmax-to-Mecl relation,
while random sampling results in statistical variations of the shape of the
IMF. The observed mmax-to-Mecl correlation and the small spread of IMF
power-law indices together suggest that optimally sampling the IMF may be the
more realistic description of star formation than random sampling from a
universal IMF with a constant upper mass limit. Composite populations on galaxy
scales, which are formed from many pc scale star formation events, need to be
described by the integrated galactic IMF. This IGIMF varies systematically from
top-light to top-heavy in dependence of galaxy type and star formation rate,
with dramatic implications for theories of galaxy formation and evolution.Comment: 167 pages, 37 figures, 3 tables, published in Stellar Systems and
Galactic Structure, Vol.5, Springer. This revised version is consistent with
the published version and includes additional references and minor additions
to the text as well as a recomputed Table 1. ISBN 978-90-481-8817-
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201