53 research outputs found
HI-deficient galaxies in intermediate density environments
Observations show that spiral galaxies in galaxy clusters tend to have on
average less neutral hydrogen (HI) than galaxies of the same type and size in
the field. There is accumulating evidence that such HI-deficient galaxies are
also relatively frequent in galaxy groups. An important question is, which
mechanisms are responsible for the gas deficiency in galaxy groups. To gain a
better understanding of how environment affects the gas content of galaxies, we
identified a sample of six HI-deficient galaxies from the HI Parkes All Sky
Survey (HIPASS) using HI-optical scaling relations. One of the galaxies is
located in the outskirts of the Fornax cluster, four are in loose galaxy groups
and one is in a galaxy triplet. We present new high resolution HI observations
with the Australia Telescope Compact Array (ATCA) of these galaxies. We discuss
the possible cause of HI-deficiency in these galaxies based on HI observations
and various multi-wavelength data. We find that the galaxies have truncated HI
disks, lopsided gas distribution and some show asymmetries in their stellar
disks. We conclude that both ram pressure stripping and tidal interactions are
important gas removal mechanisms in low density environments.Comment: Accepted to MNRAS, 17 pages, 10 figures, 7 table
H I-deficient galaxies in intermediate-density environments
Observations show that spiral galaxies in galaxy clusters tend to have on average less neutral hydrogen (H I) than galaxies of the same type and size in the field. There is accumulating evidence that such H I-deficient galaxies are also relatively frequent in galaxy groups. An important question is that which mechanisms are responsible for the gas deficiency in galaxy groups. To gain a better understanding of how environment affects the gas content of galaxies, we identified a sample of six H I-deficient galaxies from the H I Parkes All Sky Survey (HIPASS) using H I-optical scaling relations. One of the galaxies is located in the outskirts of the Fornax cluster, four are in loose galaxy groups and one is in a galaxy triplet. We present new high-resolution H I observations with the Australia Telescope Compact Array (ATCA) of these galaxies. We discuss the possible cause of H I-deficiency in the sample based on H I observations and various multi-wavelength data. We find that the galaxies have truncated H I discs, lopsided gas distribution and some show asymmetries in their stellar discs. We conclude that both ram-pressure stripping and tidal interactions are important gas removal mechanisms in low-density environments
Gaseous Tidal Debris found in the NGC 3783 Group
We have conducted wide-field HI mapping of a ~5.5 x 5.5 degree region
surrounding the NGC 3783 galaxy group, to an HI mass limit of ~4 x 10^8 Msun.
The observations were made using the multibeam system on the Parkes 64-m
radiotelescope, as part of the Galaxy Evolution Multiwavelength Study (GEMS).
We find twelve HI detections in our Parkes data, four more than catalogued in
HIPASS. We find two new group members, and discover an isolated region of HI
gas with an HI mass of ~4 x 10^8 Msun, without a visible corresponding optical
counterpart. We discuss the likelihood of this HI region being a low surface
brightness galaxy, primordial gas, or a remnant of tidal debris. For the NGC
3783 group we derive a mean recession velocity of 2903 km/s, and a velocity
dispersion of 190 km/s. The galaxy NGC 3783 is the nearest galaxy to the
luminosity weighted centre of the group, and is at the group mean velocity.
From the X-ray and dynamical state of this galaxy group, this group appears
to be in the early stages of its evolution.Comment: 12 pages, MNRAS accepted: full resolution paper available at
http://astronomy.swin.edu.au/~vkilborn/MF1350rv.pd
Southern GEMS groups II: HI distribution, mass functions and HI deficient galaxies
We investigate the neutral hydrogen (HI) content of sixteen groups for which
we have multi-wavelength data including X-ray observations. Wide-field imaging
of the groups was obtained with the 20-cm multibeam system on the 64-m Parkes
telescope. We have detected ten previously uncatalogued HI sources, one of
which has no visible optical counterpart. We examine the HI properties of the
groups, compared to their X-ray characteristics, finding that those groups with
a higher X-ray temperature and luminosity contain less HI per galaxy. The HI
content of a group depends on its morphological make-up, with those groups
dominated by early-type galaxies containing the least total HI. We determined
the expected HI for the spiral galaxies in the groups, and found that a number
of the galaxies were HI deficient. The HI deficient spirals were found both in
groups with and without a hot intra-group medium. The HI deficient galaxies
were not necessarily found at the centre of the groups, however, we did find
that two thirds of HI deficient galaxies were found within about 1 Mpc from the
group centre, indicating that the group environment is affecting the gas-loss
from these galaxies. We determined the HI mass function for a composite sample
of 15 groups, and found that it is significantly flatter than the field HI mass
function. We also find a lack of high HI-mass galaxies in groups. One possible
cause of this effect is the tidal stripping of HI gas from spiral galaxies as
they are pre-processed in groups.Comment: accepted for publication in MNRAS, 26 pages, 13 Figures, 2 Appendice
An H I Survey of Six Local Group Analogs. II. H I Properties of Group Galaxies
We have conducted an H I 21cm emission-line survey of six loose groups of galaxies chosen to be analogs to the Local Group. The survey was conducted using the Parkes multibeam instrument and the Australia Telescope Compact Array (ATCA) over a 1Mpc2 area and covering the full depth of each group, with an MH I sensitivity of 7 × 105 M. Our survey detected 110sources, 61 of which are associated with the six groups. All of these sources were confirmed with ATCA observations or were previously cataloged by HIPASS. The sources all have optical counterparts and properties consistent with dwarf irregular or late-type spiral galaxies. We present here the H I properties of the groups and their galaxies. We derive an H I mass function (HIMF) for the groups that is consistent with being flatter than the equivalent field HIMF. We also derive a circular velocity distribution function, tracing the luminous dark matter halos in the groups, that is consistent with those of the Local Group and HIPASS galaxies, both of which are shallower than that of clusters or predictions from cold dark matter models of galaxy formation
The Group Evolution Multiwavelength Study (GEMS): the Sample and Datasets
Galaxy groups have been under-studied relative to their richer counterparts
-- clusters. The Group Evolution Multiwavelength Study (GEMS) aims to redress
some the balance. Here we describe the GEMS sample selection and resulting
sample of 60 nearby (distance < 130 Mpc) galaxy groups and our multiwavelength
dataset of X-ray, optical and HI imaging. ROSAT X-ray images of each group are
presented. GEMS also utilizes near-infrared imaging from the 2MASS survey and
optical spectra from the 6dFGS. These observational data are complemented by
mock group catalogues generated from the latest LCDM simulations with gas
physics included. Existing GEMS publications are briefly highlighted as are
future publication plans.Comment: Accepted for publication in PASA. Paper plus additional figures in
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Is NGC 3108 transforming itself from an early to late type galaxy -- an astronomical hermaphrodite?
A common feature of hierarchical galaxy formation models is the process of
"inverse" morphological transformation: a bulge dominated galaxy accretes a gas
disk, dramatically reducing the system's bulge-to-disk mass ratio. During their
formation, present day galaxies may execute many such cycles across the Hubble
diagram. A good candidate for such a "hermaphrodite" galaxy is NGC 3108: a
dust-lane early-type galaxy which has a large amount of HI gas distributed in a
large scale disk. We present narrow band H_alpha and R-band imaging, and
compare the results with the HI distribution. The emission is in two
components: a nuclear bar and an extended disk component which coincides with
the HI distribution. This suggests that a stellar disk is currently being
formed out of the HI gas. The spatial distributions of the H_alpha and HI
emission and the HII regions are consistent with a barred spiral structure,
extending some 20 kpc in radius. We measure an extinction- corrected SFR of
0.42 Msun/yr. The luminosity function of the HII regions is similar to other
spiral galaxies, with a power law index of -2.1, suggesting that the star
formation mechanism is similar to other spiral galaxies. We measured the
current disk mass and find that it is too massive to have been formed by the
current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was
higher in the past. With the current SFR, the disk in NGC 3108 will grow to be
~6.2x10^9 Msun in stellar mass within the next 5.5 Gyr. While this is
substantial, the disk will be insignificant compared with the large bulge mass:
the final stellar mass disk-to-bulge ratio will be ~0.02. NGC 3108 will fail to
transform into anything resembling a spiral without a boost in the SFR and
additional supply of gas.Comment: 9 pages, 3 figures, accepted for publication in MNRA
Where are the High Velocity Clouds in Local Group Analogs?
High-velocity clouds (HVCs) are clouds of HI seen around the Milky Way with
velocities inconsistent with Galactic rotation, have unknown distances and
masses and controversial origins. One possibility is that HVCs are associated
with the small dark matter halos seen in models of galaxy formation and
distributed at distances of 150 kpc - 1 Mpc. We report on our attempts to
detect the analogs to such putative extragalactic clouds in three groups of
galaxies similar to our own Local Group using the ATNF Parkes telescope and
Compact Array. Eleven dwarf galaxies were found, but no HI clouds lacking stars
were detected. Using the population of compact HVCs around the Milky Way as a
template, we find that our non-detection of analogs implies that they must be
clustered within 160 kpc of the Milky Way (and other galaxies) with an average
HI mass <4x10^5 M(sun) at the 95% confidence level. This is in accordance with
recent limits derived by other authors. If our groups are true analogs to the
Local Group, then this makes the original Blitz et al. and Braun & Burton
picture of HVCs residing out to 1 Mpc from the Milky Way extremely unlikely.
The total HI mass in HVCs, < 10^8 M(sun), implies that there is not a large
reservoir of neutral hydrogen waiting to be accreted onto the Milky Way. Any
substantial reservoir of baryonic matter must be mostly ionized or condensed
enough as to be undetectable.Comment: 5 pages, 2 figures, ApJ letters, in pres
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