214 research outputs found
Variables in Globular Cluster NGC 5024
We present the results of a commissioning campaign to observe Galactic
globular clusters for the search of microlensing events. The central 10' X 10'
region of the globular cluster NGC 5024 was monitored using the 2-m Himalayan
Chandra Telescope in R-band for a period of about 8 hours on 24 March 2010.
Light curves were obtained for nearly 10,000 stars, using a modified Difference
Image Analysis (DIA) technique. We identified all known variables within our
field of view and revised periods and status of some previously reported
short-period variables. We report about eighty new variable sources and present
their equatorial coordinates, periods, light curves and possible types. Out of
these, 16 are SX Phe stars, 10 are W UMa-type stars, 14 are probable RR Lyrae
stars and 2 are detached eclipsing binaries. Nine of the newly discovered SX
Phe stars and two eclipsing binaries belong to the Blue Straggler Star (BSS)
population.Comment: 29 pages, 22 figures, replaced with rewritten data reduction par
Searches for Ultra-Compact Dwarf Galaxies in Galaxy Groups
We present the results of a search for ultra-compact dwarf galaxies (UCDs) in
six different galaxy groups: Dorado, NGC1400, NGC0681, NGC4038, NGC4697 and
NGC5084. We searched in the apparent magnitude range 17.5 < b_j < 20.5 (except
NGC5084: 19.2 < b_j < 21.0). We found 1 definite plus 2 possible UCD candidates
in the Dorado group and 2 possible UCD candidates in the NGC1400 group. No UCDs
were found in the other groups. We compared these results with predicted
luminosities of UCDs in the groups according to the hypothesis that UCDs are
globular clusters formed in galaxies. The theoretical predictions broadly agree
with the observational results, but deeper surveys are needed to fully test the
predictions.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Formation of ultra-compact dwarf galaxies: tests of the galaxy threshing scenario in Fornax
This paper investigates the possibility that UCD galaxies in the Fornax
cluster are formed by the threshing of nucleated, early-type dwarf galaxies
(hereafter dwarf galaxies).
Similar to the results of Cote et al. (2006) for the Virgo cluster, we show
that the Fornax Cluster observations are consistent with a single population in
which all dwarfs are nucleated, with a ratio of nuclear to total magnitude that
varies slowly with magnitude. Importantly, the magnitude distribution of the
UCD population is similar to that of the dwarf nuclei in the Fornax cluster.
The joint population of UCDs and the dwarfs from which they may originate is
modelled and shown to be consistent with an NFW profile with a characteristic
radius of 5 kpc. Furthermore, a steady-state dynamical model reproduces the
known mass profile of Fornax. However, there are a number of peculiarities in
the velocity dispersion data that remain unexplained.
The simplest possible threshing model is tested, in which dwarf galaxies move
on orbits in a static cluster potential and are threshed if they pass within a
radius at which the tidal force from the cluster exceeds the internal gravity
at the core of their dark matter halo. This fails to reproduce the observed
fraction of UCDs at radii greater than 30 kpc from the core of Fornax.Comment: 18 pages, accepted for publication by MNRAS Changes in response to
referee's comments: Amended Figure 6 to allow for missing UCDs at large radii
Modified discussio
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
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&
Structural properties of ultra-compact dwarf galaxies in the Fornax and Virgo Clusters
We present a detailed analysis of high-resolution two-band Hubble Space Telescope Advanced Camera for Surveys imaging of 21 ultra-compact dwarf (UCD) galaxies in the Virgo and Fornax Clusters. The aim of this work is to test two formation hypotheses for UCDsâwhether they are bright globular clusters (GCs) or stripped ("threshed") early-type dwarf galaxiesâby direct comparison of UCD structural parameters and colors with GCs and galaxy nuclei. We find that the UCD surface brightness profiles can be described by a range of models and that the luminous UCDs in particular cannot be described by standard King models with tidal cutoffs as they have extended outer halos. This is not expected from traditional King models of GCs, but is consistent with recent results for massive GCs. The total luminosities, colors, and sizes of the UCDs (their position in the color-magnitude and luminosity-size diagrams) are consistent with them being either luminous GCs or threshed nuclei of both early-type and late-type galaxies (not just early-type dwarfs). For the most luminous UCDs we estimate color gradients over a limited range of radius. These are systematically positive in the sense of getting redder outward: mean Î(F606W â F814W) = 0.14 mag per 100 pc with rms = 0.06 mag per 100 pc. The positive gradients found in the bright UCDs are consistent with them being either bright GCs or threshed early-type dwarf galaxies (except VUCD3). In contrast to the above results we find a very significant (>99.9% significance) difference in the sizes of UCDs and early-type galaxy nuclei: the effective radii of UCDs are 2.2+0.2 â0.1 times larger than those of early-type galaxy nuclei at the same luminosity. This result suggests that an important test can be made of the threshing hypothesis by simulating the process and predicting what size increase is expected
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