179 research outputs found
An Extragalactic HI Cloud with No Optical Counterpart?
We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of an
isolated cloud of neutral hydrogen which we believe to be extragalactic. The HI
mass of the cloud (HIPASS J1712-64) is very low, 1.7 x 10^7 Msun, using an
estimated distance of ~3.2 Mpc. Most significantly, we have found no optical
companion to this object to very faint limits (mu(B)~ 27 mag arcsec^-2). HIPASS
J1712-64 appears to be a binary system similar to, but much less massive than,
HI 1225+01 (the Virgo HI Cloud) and has a size of at least 15 kpc. The mean
velocity dispersion, measured with the Australia Telescope Compact Array
(ATCA), is only 4 km/s for the main component and because of the weak or
non-existent star-formation, possibly reflects the thermal linewidth (T<2000 K)
rather than bulk motion or turbulence. The peak column density for HIPASS
J1712-64, from the combined Parkes and ATCA data, is only 3.5 x 10^19 cm^-2,
which is estimated to be a factor of two below the critical threshold for star
formation. Apart from its significantly higher velocity, the properties of
HIPASS J1712-64 are similar to the recently recognised class of Compact High
Velocity Clouds. We therefore consider the evidence for a Local Group or
Galactic origin, although a more plausible alternative is that HIPASS J1712-64
was ejected from the interacting Magellanic Cloud/Galaxy system at
perigalacticon ~ 2 x 10^8 yr ago.Comment: 23 pages, 7 figures, AJ accepte
The disruption of nearby galaxies by the Milky Way
Interactions between galaxies are common and are an important factor in
determining their physical properties such as position along the Hubble
sequence and star-formation rate. There are many possible galaxy interaction
mechanisms, including merging, ram-pressure stripping, gas compression,
gravitational interaction and cluster tides. The relative importance of these
mechanisms is often not clear, as their strength depends on poorly known
parameters such as the density, extent and nature of the massive dark halos
that surround galaxies. A nearby example of a galaxy interaction where the
mechanism is controversial is that between our own Galaxy and two of its
neighbours -- the Large and Small Magellanic Clouds. Here we present the first
results of a new HI survey which provides a spectacular view of this
interaction. In addition to the previously known Magellanic Stream, which
trails 100 degrees behind the Clouds, the new data reveal a counter-stream
which lies in the opposite direction and leads the motion of the Clouds. This
result supports the gravitational model in which leading and trailing streams
are tidally torn from the body of the Magellanic Clouds.Comment: 17 pages with 5 figures in gif format, scheduled for publication in
the August 20th, 1998 issue of Natur
HIPASS High-Velocity Clouds: Properties of the Compact and Extended Populations
A catalog of Southern anomalous-velocity HI clouds at Decl. < +2 deg is
presented, based on data from the HI Parkes All-Sky Survey (HIPASS). The
improved sensitivity (5sigma: T_B = 0.04 K) and resolution (15.5') of the
HIPASS data results in a substantial increase in the number of individual
clouds (1956, as well as 41 galaxies) compared to previous surveys. Most
high-velocity emission features, HVCs, have a filamentary morphology and are
loosely organized into large complexes extending over tens of degrees. In
addition, 179 compact and isolated anomalous-velocity objects, CHVCs, are
identified based on their size and degree of isolation. 25% of the CHVCs
originally classified by Braun & Burton (1999) are reclassified. Both the
entire population of high-velocity emission features and the CHVCs alone have
typical HI masses of ~ 4.5 D(kpc)^2 solar masses and have similar slopes for
their column density and flux distributions. On the other hand, the CHVCs
appear to be clustered and the population can be broken up into three spatially
distinct groups, while the entire population of clouds is more uniformly
distributed with a significant percentage aligned with the Magellanic Stream.
The median velocities are V_GSR = -38 km/s for the CHVCs and -30 km/s for all
of the anomalous-velocity clouds. Based on the catalog sizes, high-velocity
features cover 19% of the southern sky and CHVCs cover 1%. (abridged)Comment: 32 pages, 26 figures in gif format, 2 ascii tables, to appear in the
Jan 2002 issue of The Astronomical Journal, high resolution version available
at http://origins.Colorado.EDU/~mputman/pubs.htm
The Large Scale Distribution of Neutral Hydrogen in the Fornax Region
Using HIPASS data, we have searched for HI in a ~25x25 sq.deg. region centred
on the Fornax cluster. Within a velocity search range of 300 - 3700 km/s and a
lower flux limit of ~40 mJy, 110 galaxies with HI emission were detected, one
of which is previously uncatalogued. None of the detections has early-type
morphology. Previously unknown velocities for 14 galaxies have been determined,
with a further 4 velocity measurements being significantly dissimilar to
published values. Identification of an optical counterpart is relatively
unambiguous for more than ~90% of our HI galaxies. The galaxies appear to be
embedded in a sheet at the cluster velocity which extends for more than 30 deg
across the search area. At the nominal cluster distance of ~20 Mpc, this
corresponds to an elongated structure more than 10 Mpc in extent. A velocity
gradient across the structure is detected, with radial velocities increasing by
\~500 km/s from SE to NW. The clustering of galaxies evident in optical surveys
is only weakly suggested in the spatial distribution of our HI detections. Our
results suggest a considerable deficit of HI-rich galaxies in the centre of the
cluster. However, relative to the field, there is a 3(+/-1)-fold excess of
HI-rich galaxies in the outer parts of the cluster where galaxies may be
infalling towards the cluster for the first time.Comment: 16 pages, 14 figures, 110 HI spectra. To be published in MNRA
New Galaxies Discovered in the First Blind HI Survey of the Centaurus A Group
We have commenced a 21-cm survey of the entire southern sky (\delta < 0
degrees, -1200 km/s < v < 12700 km/s) which is ''blind'', i.e. unbiased by
previous optical information. In the present paper we report on the results of
a pilot project which is based on data from this all-sky survey. The project
was carried out on an area of 600 square degrees centred on the nearby
Centaurus A (Cen A) group of galaxies at a mean velocity of v ~ 500 km/s. This
was recently the subject of a separate and thorough optical survey.
We found 10 new group members to add to the 21 galaxies already known in the
Cen A group: five of these are previously uncatalogued galaxies, while five
were previously catalogued but not known to be associated with the group.
We found optical counterparts for all the HI detections, most of them
intrinsically very faint low surface brightness dwarfs. The new group members
add approximately 6% to the HI mass of the group and 4% to its light. The HI
mass function, derived from all the known group galaxies in the interval 10^7
\Msun of HI to 10^9 \Msun of HI, has a faint-end slope of 1.30 +/- 0.15,
allowing us to rule out a slope of 1.7 at 95% confidence. Even if the number in
the lowest mass bin is increased by 50%, the slope only increases to 1.45 +/-
0.15.Comment: 19 pages Latex, 6 figures (Fig. 2 in four parts, Fig.5 in two parts).
To appear in The Astrophysical Journal (Vol. 524, October 1999
The HIX galaxy survey II: HI kinematics of HI eXtreme galaxies
By analysing a sample of galaxies selected from the HI Parkes All Sky Survey
(HIPASS) to contain more than 2.5 times their expected HI content based on
their optical properties, we investigate what drives these HI eXtreme (HIX)
galaxies to be so HI-rich. We model the HI kinematics with the Tilted Ring
Fitting Code TiRiFiC and compare the observed HIX galaxies to a control sample
of galaxies from HIPASS as well as simulated galaxies built with the
semi-analytic model Dark Sage. We find that (1) HI discs in HIX galaxies are
more likely to be warped and more likely to host HI arms and tails than in the
control galaxies, (2) the average HI and average stellar column density of HIX
galaxies is comparable to the control sample, (3) HIX galaxies have higher HI
and baryonic specific angular momenta than control galaxies, (4) most HIX
galaxies live in higher-spin haloes than most control galaxies. These results
suggest that HIX galaxies are HI-rich because they can support more HI against
gravitational instability due to their high specific angular momentum. The
majority of the HIX galaxies inherits their high specific angular momentum from
their halo. The HI content of HIX galaxies might be further increased by
gas-rich minor mergers. This paper is based on data obtained with the Australia
Telescope Compact Array (ATCA) through the large program C 2705.Comment: 14 pages, 7 figures plus Appendix, published in MNRAS; Erratum to be
published in MNRAS: correction of table values in Tab.3 and 4 (the latest
arXiv submission contains the correct table values
Intergalactic HII Regions Discovered in SINGG
A number of very small isolated HII regions have been discovered at projected
distances up to 30 kpc from their nearest galaxy. These HII regions appear as
tiny emission line objects in narrow band images obtained by the NOAO Survey
for Ionization in Neutral Gas Galaxies (SINGG). We present spectroscopic
confirmation of four isolated HII regions in two systems, both systems have
tidal HI features. The results are consistent with stars forming in interactive
debris due to cloud-cloud collisions. The H-alpha luminosities of the isolated
HII regions are equivalent to the ionizing flux of only a few O stars each.
They are most likely ionized by stars formed in situ, and represent atypical
star formation in the low density environment of the outer parts of galaxies. A
small but finite intergalactic star formation rate will enrich and ionize the
surrounding medium. In one system, NGC 1533, we calculate a star formation rate
of 1.5e-3 msun/yr, resulting in a metal enrichment of ~1e-3 solar for the
continuous formation of stars. Such systems may have been more common in the
past and a similar enrichment level is measured for the `metallicity floor' in
damped Lyman-alpha absorption systems.Comment: accepted for publication in the Astronomical Journal, 19 pages,
including 5 figures, some low resolution. Paper with high resolution images
can be downloaded from
http://astro.ph.unimelb.edu.au/~eryan/publications/eldots.ps.g
The Survey for Ionization in Neutral Gas Galaxies- II. The Star Formation Rate Density of the Local Universe
We derive observed Halpha and R band luminosity densities of an HI-selected
sample of nearby galaxies using the SINGG sample to be l_Halpha' = (9.4 +/-
1.8)e38 h_70 erg s^-1 Mpc^-3 for Halpha and l_R' = (4.4 +/- 0.7)e37 h_70 erg
s^-1 A^-1 Mpc^-3 in the R band. This R band luminosity density is approximately
70% of that found by the Sloan Digital Sky Survey. This leads to a local star
formation rate density of log(SFRD) = -1.80 +0.13/-0.07(random) +/-
0.03(systematic) + log(h_70) after applying a mean internal extinction
correction of 0.82 magnitudes. The gas cycling time of this sample is found to
be t_gas = 7.5 +1.3/-2.1 Gyr, and the volume-averaged equivalent width of the
SINGG galaxies is EW(Halpha) = 28.8 +7.2/-4.7 A (21.2 +4.2/-3.5 A without
internal dust correction). As with similar surveys, these results imply that
SFRD(z) decreases drastically from z ~ 1.5 to the present. A comparison of the
dynamical masses of the SINGG galaxies evaluated at their optical limits with
their stellar and HI masses shows significant evidence of downsizing: the most
massive galaxies have a larger fraction of their mass locked up in stars
compared with HI, while the opposite is true for less massive galaxies. We show
that the application of the Kennicutt star formation law to a galaxy having the
median orbital time at the optical limit of this sample results in a star
formation rate decay with cosmic time similar to that given by the SFRD(z)
evolution. This implies that the SFRD(z) evolution is primarily due to the
secular evolution of galaxies, rather than interactions or mergers. This is
consistent with the morphologies predominantly seen in the SINGG sample.Comment: 15 pages, 5 figures, ApJ in press. Data available at
http://sungg.pha.jhu.edu/PubData/ Corrected: Minor typos and formatting
issues fixe
The 1000 Brightest HIPASS Galaxies: HI Mass Function and Omega_HI
We present a new accurate measurement of the HI mass function of galaxies
from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the
highest HI peak flux densities in the southern hemisphere (Koribalski et al.
2003). This sample spans nearly four orders of magnitude in HI mass (from log
M_HI/M_sun=6.8 to 10.6, H0=75) and is the largest sample of HI selected
galaxies to date. We develop a bivariate maximum likelihood technique to
measure the space density of galaxies, and show that this is a robust method,
insensitive to the effects of large scale structure. The resulting HI mass
function can be fitted satisfactorily with a Schechter function with faint-end
slope alpha=-1.30. This slope is found to be dependent on morphological type,
with later type galaxies giving steeper slopes. We extensively test various
effects that potentially bias the determination of the HI mass function,
including peculiar motions of galaxies, large scale structure, selection bias,
and inclination effects, and quantify these biases. The large sample of
galaxies enables an accurate measurement of the cosmological mass density of
neutral gas: Omega_HI=(3.8 +/- 0.6) x 10^{-4}. Low surface brightness galaxies
contribute only 15% to this value, consistent with previous findings.Comment: accepted for publication in Astronomical Journal, 16 pages, including
17 figures. Corrected typos and reference
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