137 research outputs found
The HI Content of Compact Groups of Galaxies
The HI content of Hickson Compact Groups in the southern hemisphere is
measured using data from the HI Parkes All Sky Survey (HIPASS), and dedicated
observations using the narrowband filter on the Multibeam instrument on the
Parkes telescope. The expected HI mass of these groups was estimated using the
luminosity, diameter and morphological types of the member galaxies, calibrated
from published data. Taking careful account of non-detection limits, the
results show that the compact group population that has been detected by these
observations has an HI content similar to that of galaxies in the reference
field sample. The upper limits for the undetected groups lie within the normal
range; improvement of these limits will require a large increase in
sensitivity.Comment: 27 pages, 5 figures. Accepted for publication in PAS
NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey
We present an HI map constructed from the Arecibo Legacy Fast ALFA (ALFALFA)
survey of the surroundings of the strongly asymmetric Virgo cluster Sc galaxy
NGC 4254. Noted previously for its lopsided appearance, rich interstellar
medium, and extradisk HI emission, NGC 4254 is believed to be entering the
Virgo environment for the first time and at high speed. The ALFALFA map clearly
shows a long HI tail extending ~250 kpc northward from the galaxy. Embedded as
one condensation within this HI structure is the object previously identified
as a "dark galaxy": Virgo HI21 (Davies et al. 2004). A body of evidence
including its location within and velocity with respect to the cluster and the
appearance and kinematics of its strong spiral pattern, extra-disk HI and
lengthy HI tail is consistent with a picture of "galaxy harassment" as proposed
by Moore et al. (1996a,b; 1998). The smoothly varying radial velocity field
along the tail as it emerges from NGC 4254 can be used as a timing tool, if
interpreted as resulting from the coupling of the rotation of the disk and the
collective gravitational forces associated with the harassment mechanism.Comment: accepted for publication in Ap.J.(Lett.). higher resolution figure
available at http://egg.astro.cornell.edu/alfalfa/pubs/figs/n4254_f1.ep
HI content in galaxies in loose groups
Gas deficiency in cluster spirals is well known and ram-pressure stripping is
considered the main gas removal mechanism. In some compact groups too gas
deficiency is reported. However, gas deficiency in loose groups is not yet well
established. Lower dispersion of the member velocities and the lower density of
the intra-group medium in small loose groups favour tidal stripping as the main
gas removal process in them. Recent releases of data from HI Parkes all sky
survey (HIPASS) and catalogues of nearby loose groups with associated diffuse
X-ray emission have allowed us to test this notion. In this paper, we address
the following questions: (a) do galaxies in groups with diffuse X-ray emission
statistically have lower gas content compared to the ones in groups without
diffuse X-ray emission? (b) does HI deficiency vary with the X-ray luminosity
of the loose group in a systematic way? We find that (a) galaxies in groups
with diffuse X-ray emission, on average, are HI deficient, and have lost more
gas compared to those in groups without X-ray emission; the later are found not
to have significant HI deficiency; (b) no systematic dependence of the HI
deficiency with X-ray luminosity is found. Ram pressure assisted tidal
stripping and evaporation by thermal conduction are the two possible mechanisms
to account for this excess gas loss.Comment: 9 pages, 4 figures, accepted in MNRA
The Cold and Hot Gas Content of Fine-Structure E and S0 Galaxies
We investigate trends of the cold and hot gas content of early-type galaxies
with the presence of optical morphological peculiarities, as measured by the
fine-structure index (Sigma). HI mapping observations from the literature are
used to track the cold-gas content, and archival ROSAT PSPC data are used to
quantify the hot-gas content. We find that E and S0 galaxies with a high
incidence of optical peculiarities are exclusively X-ray underluminous and,
therefore, deficient in hot gas. In contrast, more relaxed galaxies with little
or no signs of optical peculiarities span a wide range of X-ray luminosities.
That is, the X-ray excess anticorrelates with Sigma. There appears to be no
similar trend of cold-gas content with either fine-structure index or X-ray
content. The fact that only apparently relaxed E and S0 galaxies are strong
X-ray emitters is consistent with the hypothesis that after strong disturbances
such as a merger hot-gas halos build up over a time scale of several gigayears.
This is consistent with the expected mass loss from stars.Comment: 12 pages, latex, 5 figures. Accepted for publication in A
Dust and Gas as Seeds for Metal-Poor Star Formation
I address the issue of dust and gas as seeds for metal-poor star formation by
reviewing what we know about star formation in nearby dwarf galaxies and its
relationship to the gas and dust. I (try to) speculate on the extent to which
processes in nearby galaxies mimic star formation in the early universe.Comment: To be published in "Low Metallicity Star Formation: From the First
Stars to Dwarf Galaxies," Proceedings of IAU Symposium No. 255, eds. L.K.
Hunt, S. Madden, & R. Schneider (Cambridge: Cambridge Univ Press
Is the far border of the Local Void expanding?
According to models of evolution in the hierarchical structure formation
scenarios, voids of galaxies are expected to expand. The Local Void (LV) is the
closest large void, and it provides a unique opportunity to test
observationally such an expansion. It has been found that the Local Group,
which is on the border of the LV, is running away from the void center at ~260
km/s. In this study we investigate the motion of the galaxies at the far-side
border of the LV to examine the presence of a possible expansion. We selected
late-type, edge-on spiral galaxies with radial velocities between 3000 km/s and
5000 km/s, and carried out HI 21 cm line and H-band imaging observations. The
near-infrared Tully-Fisher relation was calibrated with a large sample of
galaxies and carefully corrected for Malmquist bias. It was used to compute the
distances and the peculiar velocities of the LV sample galaxies. Among the 36
sample LV galaxies with good quality HI line width measurements, only 15
galaxies were selected for measuring their distances and peculiar velocities,
in order to avoid the effect of Malmquist bias. The average peculiar velocity
of these 15 galaxies is found to be -419+208-251 km/s, which is not
significantly different from zero. Due to the intrinsically large scatter of
Tully-Fisher relation, we cannot conclude whether there is a systematic motion
against the center of the LV for the galaxies at the far-side boundary of the
void. However, our result is consistent with the hypothesis that those galaxies
at the far-side boundary have an average velocity of ~260 km/s equivalent to
what is found at the position of the Local Group.Comment: 15 pages, 6 figures, and 4 tables. Accepted for publication in A&
An Overview of Uncovered and Suspected Large-Scale Structures behind the Milky Way
Various dynamically important extragalactic large-scale structures in the
local Universe lie behind the Milky Way. Most of these structures (predicted
and unexpected) have only recently been made ``visible'' through dedicated deep
surveys at various wavelengths. The wide range of observational searches
(optical, near infrared, far infrared, radio and X-ray) for galaxies in the
Zone of Avoidance (ZOA) will be reviewed and the uncovered and suspected
large-scale structures summarised. Particular emphasis is given to the Great
Attractor region where the existence of yet another cluster is suspected (Woudt
1998). Predictions from reconstructions of the density field in the ZOA are
discussed and compared with observational evidence. Although no major
structures are predicted out to about v < 10000 km/s for which no observational
evidence exists, the comparison between reconstructed density fields and the
observed galaxy distribution remain important as they allow derivations of the
density and biasing parameters.Comment: To appear in PASA 16. Proceedings of workshop "HI in the Local
Universe, II", held in Melbourne, Sept. 1998. 9 pages, LaTeX2e, 1
encapsulated PS figur
The origin of type I profiles in cluster lenticulars: an interplay between ram pressure stripping and tidally induced spiral migration
Using N-body + smooth particle hydrodynamics simulations of galaxies falling into a cluster, we study the evolution of their radial density profiles. When evolved in isolation, galaxies develop a type II (down-bending) profile. In the cluster, the evolution of the profile depends on the minimum cluster-centric radius the galaxy reaches, which controls the degree of ram pressure stripping. If the galaxy falls to ∼50 per cent of the virial radius, then the profile remains type II, but if the galaxy reaches down to ∼20 per cent of the virial radius, the break weakens and the profile becomes more type I like. The velocity dispersions are only slightly increased in the cluster simulations compared with the isolated galaxy; random motion therefore cannot be responsible for redistributing material sufficiently to cause the change in the profile type. Instead, we find that the joint action of radial migration driven by tidally induced spirals and the outside-in quenching of star formation due to ram pressure stripping alters the density profile. As a result, this model predicts a flattening of the age profiles amongst cluster lenticulars with type I profiles, which can be observationally tested
Mapping the Hidden Universe: The Galaxy Distribution in the Zone of Avoidance
Due to the foreground extinction of the Milky Way, galaxies become
increasingly faint as they approach the Galactic Equator creating a ``zone of
avoidance'' (ZOA) in the distribution of optically visible galaxies of about
25%. A ``whole-sky'' map of galaxies is essential, however, for understanding
the dynamics in our local Universe, in particular the peculiar velocity of the
Local Group with respect to the Cosmic Microwave Background and velocity flow
fields such as in the Great Attractor (GA) region. The current status of deep
optical galaxy searches behind the Milky Way and their completeness as a
function of foreground extinction will be reviewed. It has been shown that
these surveys - which in the mean time cover the whole ZOA (Fig. 2) - result in
a considerable reduction of the ZOA from extinction levels of A_B = 1.0 mag
(Fig. 1) to A_B = 3.0 mag (Fig. 2). In the remaining, optically opaque ZOA,
systematic HI surveys are powerful in uncovering galaxies, as is demonstrated
for the GA region with data from the full sensitivity Parkes Multibeam HI
survey (300 < l < 332 deg, |b| < 5.5 deg, Fig. 4).Comment: Accepted for publication in PASA (Volume 17, 1, to appear April
2000); LaTex, 4 encapsulated (reduced) ps-figures, requires psfig.
Full-resolution color figures 1, 3, and 4 are available upon request at
e-mail: [email protected] or at http://www.atnf.csiro.au/pasa/17_1
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