85 research outputs found
Angular Momentum Regulates Atomic Gas Fractions of Galactic Disks
We show that the mass fraction f_atm = 1.35*MHI/M of neutral atomic gas (HI
and He) in isolated local disk galaxies of baryonic mass M is well described by
a straightforward stability model for flat exponential disks. In the outer disk
parts, where gas at the characteristic dispersion of the warm neutral medium is
stable in the sense of Toomre (1964), the disk consists of neutral atomic gas;
conversely the inner part where this medium would be Toomre-unstable, is
dominated by stars and molecules. Within this model, f_atm only depends on a
global stability parameter q=j*sigma/(GM), where j is the baryonic specific
angular momentum of the disk and sigma the velocity dispersion of the atomic
gas. The analytically derived first-order solution f_atm = min{1,2.5q^1.12}
provides a good fit to all plausible rotation curves. This model, with no free
parameters, agrees remarkably well (+-0.2 dex) with measurements of f_atm in
isolated local disk galaxies, even with galaxies that are extremely HI-rich or
HI-poor for their mass. The finding that f_atm increases monotonically with q
for pure stability reasons offers a powerful intuitive explanation for the mean
variation of f_atm with M: in a cold dark matter universe galaxies are expected
to follow j~M^(2/3), which implies the average scaling q~M^(-1/3) and hence
f_atm~M^(-0.37), in agreement with observations.Comment: 5 pages, 3 figure
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
What do loose groups tell us about galaxy formation?
We present the results of a Parkes Multibeam HI survey of six loose groups of
galaxies analogous to the Local Group. This survey was sensitive to HI-rich
objects in these groups to below 10^7 M(sun) of HI, and was designed to search
for low mass, gas-rich satellite galaxies and potential analogs to the
high-velocity clouds seen around the Milky Way. This survey detected a total of
79 HI-rich objects associated with the six groups, half of which were new
detections. While the survey identified a small number of dwarf galaxies, no
star-free HI clouds were discovered. The HI mass function of the six groups
appears to be roughly flat as is that of the Local Group. The cumulative
velocity distribution function (CVDF) of the HI-rich halos in the six groups is
identical to that of the Local Group. Both of these facts imply that these
groups are true analogs to the Local Group and that the Local Group is not
unique in its lack of low-mass dwarf galaxies as compared to the predictions of
cold dark matter models of galaxy formation. This survey also constrains the
distance to and HI masses of the compact high-velocity clouds (CHVCs) around
the Milky Way. The lack of CHVC analog detections implies that they are
distributed within <160 kpc of the Milky Way and have average HI masses of
<4x10^5 M(sun). The spatial distribution of CHVCs is consistent with the
predictions of simulations for dark matter halos. Furthermore the CVDF of Local
Group galaxies plus CHVCs matches the predicted CVDF of cold dark matter
simulations of galaxy formation. This provides circumstantial evidence that
CHVCs may be associated with low-mass dark matter halos.Comment: 4 pages, to appear in the proceedings of "Baryons in Dark Matter
Halos" Eds R-J., Dettmar, U. Klein, P. Salucci, PoS, SISSA,
http://pos.sissa.i
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
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