73 research outputs found
Galaxy threshing and the formation of ultra-compact dwarf galaxies
Recent spectroscopic and morphological observational studies of galaxies
around NGC 1399 in the Fornax Cluster (Drinkwater et al. 2000b) have discovered
several `ultra-compact dwarf' galaxies with intrinsic sizes of 100 pc
and absolute band magnitudes ranging from -13 to -11 mag. In order to
elucidate the origin of these enigmatic objects, we perform numerical
simulations on the dynamical evolution of nucleated dwarf galaxies orbiting NGC
1399 and suffering from its strong tidal gravitational field. Adopting a
plausible scaling relation for dwarf galaxies, we find that the outer stellar
components of a nucleated dwarf are totally removed. This is due to them being
tidally stripped over the course of several passages past the central region of
NGC 1399. The nucleus, however, manages to survive. We also find that the size
and luminosity of the remnant are similar to those observed for ultra-compact
dwarf galaxies, if the simulated precursor nucleated dwarf has a mass of
. These results suggest that ultra-compact dwarf galaxies
could have previously been more luminous dwarf spheroidal or elliptical
galaxies with rather compact nuclei.Comment: 9 pages 4 figures,2001, ApJL, 552, 10
The Dwarf Galaxy Population of the Dorado group down to Mv=-11
We present V and I CCD photometry of suspected low-surface brightness dwarf
galaxies detected in a survey covering ~2.4 deg^2 around the central region of
the Dorado group of galaxies. The low-surface brightness galaxies were chosen
based on their sizes and magnitudes at the limiting isophote of 26.0V\mu. The
selected galaxies have magnitudes brighter than V=20 (Mv=-11 for an assumed
distance to the group of 17.2 Mpc), with central surface brightnesses \mu0>22.5
V mag/arcsec^2, scale lengths h>2'', and diameters > 14'' at the limiting
isophote. Using these criteria, we identified 69 dwarf galaxy candidates. Four
of them are large very low-surface brightness galaxies that were detected on a
smoothed image, after masking high surface brightness objects. Monte Carlo
simulations performed to estimate completeness, photometric uncertainties and
to evaluate our ability to detect extended low-surface brightness galaxies show
that the completeness fraction is, on average, > 80% for dwarf galaxies with
and 22.5<\mu0<25.5 V mag/arcsec^2, for the range of sizes
considered by us (D>14''). The V-I colors of the dwarf candidates vary from
-0.3 to 2.3 with a peak on V-I=0.98, suggesting a range of different stellar
populations in these galaxies. The projected surface density of the dwarf
galaxies shows a concentration towards the group center similar in extent to
that found around five X-ray groups and the elliptical galaxy NGC1132 studied
by Mulchaey and Zabludoff (1999), suggesting that the dwarf galaxies in Dorado
are probably physically associated with the overall potential well of the
group.Comment: 32 pages, 16 postscript figures and 3 figures in GIF format, aastex
v5.0. To appear in The Astronomical Journal, January 200
Galaxy Orientations in the Coma Cluster
We have examined the orientations of early-type galaxies in the Coma cluster
to see whether the well-established tendency for brightest cluster galaxies to
share the same major axis orientation as their host cluster also extends to the
rest of the galaxy population. We find no evidence of any preferential
orientations of galaxies within Coma or its surroundings. The implications of
this result for theories of the formation of clusters and galaxies
(particularly the first-ranked members) are discussed.Comment: Accepted for publication in the Astrophysical Journal Letters. 4
pages, 4 figure
The Principal Axis of the Virgo Cluster
Using accurate distances to individual Virgo cluster galaxies obtained by the
method of Surface Brightness Fluctuations, we show that Virgo's brightest
ellipticals have a remarkably collinear arrangement in three dimensions. This
axis, which is inclined by 10 to 15 degrees from the line of sight, can be
traced to even larger scales where it appears to join a filamentary bridge of
galaxies connecting Virgo to the rich cluster Abell 1367. The orientations of
individual Virgo ellipticals also show some tendency to be aligned with the
cluster axis, as does the jet of the supergiant elliptical M87. These results
suggest that the formation of the Virgo cluster, and its brightest member
galaxies, have been driven by infall of material along the Virgo-A1367
filament.Comment: 8 pages, 4 figures, accepted for publication in ApJ Letter
The Dwarf Irregular Galaxy UGC 7636 Exposed: Stripping At Work In The Virgo Cluster
We present the results of optical spectroscopy of a newly discovered H II
region residing in the H I gas cloud located between the dwarf irregular galaxy
UGC 7636 and the giant elliptical galaxy NGC 4472 in the Virgo Cluster. By
comparing UGC 7636 with dwarf irregular galaxies in the field, we show that the
H I cloud must have originated from UGC 7636 because (1) the oxygen abundance
of the cloud agrees with that expected for a galaxy with the blue luminosity of
UGC 7636, and (2) M_{H I}/L_B for UGC 7636 becomes consistent with the measured
oxygen abundance of the cloud if the H I mass of the cloud is added back into
UGC 7636. It is likely that tides from NGC 4472 first loosened the H I gas,
after which ram-pressure stripping removed the gas from UGC 7636.Comment: 12 pages, 2 eps figures (AASTeX 5.0); accepted for publication in ApJ
Letter
Transient allograft dysfunction from immune reconstitution in a patient with polyoma BK-virus-associated nephropathy
Alignments of the Dominant Galaxies in Poor Clusters
We have examined the orientations of brightest cluster galaxies (BCGs) in
poor MKW and AWM clusters and find that, like their counterparts in richer
Abell clusters, poor cluster BCGs exhibit a strong propensity to be aligned
with the principal axes of their host clusters as well as the surrounding
distribution of nearby (< 20/h Mpc) Abell clusters. The processes responsible
for dominant galaxy alignments are therefore independent of cluster richness.
We argue that these alignments most likely arise from anisotropic infall of
material into clusters along large-scale filaments.Comment: 8 pages, 5 figure
The Globular Cluster System of the Virgo Dwarf Elliptical Galaxy VCC 1087
We have analysed the globular cluster (GC) system of the nucleated dwarf
elliptical galaxy VCC 1087 in the Virgo cluster, based on Keck/LRIS
spectroscopy and archival HST/ACS imaging. We estimate VCC 1087 hosts a total
population of 77+/-19 GCs, which corresponds to a relatively high V-band
specific frequency of 5.8+/-1.4. The g-z color distribution of the GCs shows a
blue (metal-poor) peak with a tail of redder (metal-rich) clusters similar in
color to those seen in luminous ellipticals. Spectroscopy of a subsample of 12
GCs suggests that the GC system is old and coeval (~10 Gyr), with a fairly
broad metallicity distribution (-1.8<[m/H]<-0.8). In contrast, an integrated
spectrum of the underlying galaxy starlight reveals that its optical luminosity
is dominated by metal-rich, intermediate-aged stars. Radial velocities of the
GCs suggest rotation close to the major axis of the galaxy, and this rotation
is dynamically significant with (v/sigma)^* >1. A compilation of the kinematics
of the GC systems of 9 early-type galaxies shows surprising diversity in the
v/sigma parameter for GC systems. In this context, the GC system of VCC 1087
exhibits the most significant rotation to velocity dispersion signature.
Modeling the velocity dispersion profile of the GCs and galaxy stars suggest
fairly constant mass-to-light ratios of ~3 out to 6.5 kpc. The present
observations can entertain both baryonic and non-baryonic solutions, and GC
velocities at larger radii would be most valuable with regard to this issue. We
discuss the evolution of VCC 1087 in terms of the galaxy ``harassment''
scenario, and conclude that this galaxy may well be the remains of a faded,
tidally perturbed Sc spiral [abridged].Comment: 17 pages, 13 figures, to appear in the A
Chandra Observations of the Interacting NGC 4410 Galaxy Group
We present high resolution X-ray imaging data from the ACIS-S instrument on
the Chandra telescope of the nearby interacting galaxy group NGC 4410. Four
galaxies in the inner portion of this group are clearly detected by Chandra,
including the peculiar low luminosity radio galaxy NGC 4410A. In addition to a
nuclear point source, NGC 4410A contains diffuse X-ray emission, including an
X-ray ridge extending out to about 12" (6 kpc) to the northwest of the nucleus.
This ridge is coincident with an arc of optical emission-line gas, which has
previously been shown to have optical line ratios consistent with shock
ionization. This structure may be due to an expanding superbubble of hot gas
caused by supernovae and stellar winds or by the active nucleus. The Chandra
observations also show four or five possible compact ultra-luminous X-ray (ULX)
sources (L(x) >= 10^39 erg/s) associated with NGC 4410A. At least one of these
candidate ULXs appears to have a radio counterpart, suggesting that it may be
due to an X-ray binary with a stellar-mass black hole, rather than an
intermediate mass black hole. In addition, a faint diffuse intragroup X-ray
component has been detected between the galaxies (L(x) ~ 10^41 erg/s). This
supports the hypothesis that the NGC 4410 group is in the process of evolving
via mergers from a spiral-dominated group (which typically have no
X-ray-emitting intragroup gas) to an elliptical-dominated group (which often
have a substantial intragroup medium).Comment: 27 pages, 14 figures; Accepted by Astronomical Journal; color images
at http://www.etsu.edu/physics/bsmith/research/n4410.htm
Infrared Properties of Star Forming Dwarf Galaxies: Blue Compact Dwarfs in the Virgo Cluster
A sample of 16 blue compact dwarf galaxies (BCDs) in the Virgo Cluster has
been imaged in the near-infrared (NIR) in and on the 2.1m telescope
at OAN-SPM in Mexico. Isophotes as faint as = 24 mag arcsec and
= 23 mag arcsec have been reached in most of the targets.
Surface brightness profiles can be fitted across the whole range of radii by
the sum of two components: a hyperbolic secant (sech) function, which is known
to fit the light profiles of dIs, and a Gaussian component, which quantifies
the starburst near the centre. Isophotal and total fitted NIR magnitudes have
been calculated, along with semimajor axes at = 23 mag arcsec
and = 22 mag arcsec. The diffuse underlying component and
the young starburst have been quantified using the profile fitting. Most color
profiles show a constant color, between =0.7 to 0.9 mag. The diffuse
component represents the overwhelming majority of the NIR light for most BCDs,
the starburst enhancing the flux by less than about 0.3 mag. Linear
correlations were found between the sech scale length and the sech magnitude,
and between the sech semimajor axis and the sech magnitude. Overall, galaxies
with more luminous diffuse components are larger and brighter in the centre.
The central burst correlates with the diffuse component, with brighter BCDs
having stronger star-bursts, suggesting that more massive objects are forming
stars more efficiently. BCDs lie on the ``fundamental plane'' defined by dwarf
irregulars (dISs) in Paper I, following the same relation between sech absolute
magnitude, sech central surface brightness, and the hydrogen line-width
, although the scatter is larger than for the dIs. [one sentence cut]Comment: Accepted in A
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