494 research outputs found
IC 4200: a gas-rich early-type galaxy formed via a major merger
We present the result of radio and optical observations of the S0 galaxy IC
4200. We find that the galaxy hosts 8.5 billion solar masses of HI rotating on
a ~90 deg warped disk extended out to 60 kpc from the centre of the galaxy.
Optical spectroscopy reveals a simple-stellar-population-equivalent age of 1.5
Gyr in the centre of the galaxy and V- and R-band images show stellar shells.
Ionised gas is observed within the stellar body and is kinematically decoupled
from the stars and characterised by LINER-like line ratios.We interpret these
observational results as evidence for a major merger origin of IC 4200, and
date the merger back to 1-3 Gyr ago.Comment: Accepted for publication in Astronomy & Astrophysics; 18 pages, 13
figures; the tables of Appendix C can be downloaded at
http://www.astro.rug.nl/~pserra/IC420
Star Formation Thresholds in Galactic Disks
We report the first results of a detailed study of the star formation law in
a sample of 32 nearby spiral galaxies with well-measured rotation curves, HI
and H (as traced by CO) surface density profiles, and new \Ha CCD
photometry. Our results strongly support the view that the formation of
gravitationally bound interstellar clouds regulates the onset of widespread
star formation -- at least in the outer regions of galactic disks.Comment: Will appear in July 1 ApJ. Abbreviated abstract. Postscript version
available at http://www.astro.caltech.edu/~clm
Neutral Gas Distribution and Kinematics of the Nearly Face-on Spiral Galaxy NGC 1232
We have analyzed high velocity resolution HI synthesis observations of the
nearly face-on Sc galaxy NGC 1232. The neutral gas distribution extends well
beyond the optical extent of the galaxy. As expected, local peaks in the HI
column density are associated with the spiral arms. Further, the HI column
density drops precipitously near the center of the galaxy. Closed contours in
the velocity field suggest either that the system is warped, or that the
rotation curve declines. The velocity dispersion is approximately constant
throughout the system, with a median value of 9.9 +/- 1.8 km/s. When corrected
for rotational broadening, there is no indication of a radial trend in the
neutral gas velocity dispersion in this galaxy.Comment: 14 pages of text, 10 pages of figures. Accepted to the A
Kinematics of diffuse ionized gas in the disk halo interface of NGC 891 from Fabry-P\'erot observations
The properties of the gas in halos of galaxies constrain global models of the
interstellar medium. Kinematical information is of particular interest since it
is a clue to the origin of the gas. Here we report observations of the
kinematics of the thick layer of the diffuse ionized gas in NGC 891 in order to
determine the rotation curve of the halo gas. We have obtained a Fabry-P\'erot
data cube in Halpha to measure the kinematics of the halo gas with angular
resolution much higher than obtained from HI 21 cm observations. The data cube
was obtained with the TAURUS II spectrograph at the WHT on La Palma. The
velocity information of the diffuse ionized gas extracted from the data cube is
compared to model distributions to constrain the distribution of the gas and in
particular the halo rotation curve. The best fit model has a central
attenuation tau_H-alpha=6, a dust scale length of 8.1 kpc, an ionized gas scale
length of 5.0 kpc. Above the plane the rotation curve lags with a vertical
gradient of -18.8 km/s/kpc. We find that the scale length of the H-alpha must
be between 2.5 and 6.5 kpc. Furthermore we find evidence that the rotation
curve above the plane rises less steeply than in the plane. This is all in
agreement with the velocities measured in the HI.Comment: A&A, in press. 13 pages, 19 figure
The Discovery of a Molecular Complex in the Tidal Arms near NGC 3077
We present the discovery of a giant molecular complex (r=350 pc, M_ vir=10^7
M_sun) in the tidal arms south-east of NGC 3077, a member of the M 81 triplet.
The complex is clearly detected in the 12CO (J=1-0) transition at five
independent positions. The position relative to NGC 3077, the systemic velocity
(v_hel=14km/s) and the cloud averaged line width (15km/s) indicate that the
object is not related to Galactic cirrus but is extragalactic. The tidal HI arm
where the molecular complex is located has an total HI mass of M_HI=3x10^8
M_sun. This tidal material was presumably stripped off the outer parts of NGC
3077 during the closest encounter with M 81, about 3x10^8 years ago. After the
complex detected along a torn-out spiral arm of M 81 by Brouillet et al., it is
the second of its kind reported so far. Based on published optical
observations, we have no evidence for on--going star formation in the newly
detected molecular complex. Since the system has all the ingredients to form
stars in the future, we speculate that it might eventually resemble the young
dwarf galaxies in the M 81 group.Comment: 12 pages (including 3 figures), accepted for publication in the ApJ
Letter
Cost-effectiveness of HIV screening of blood donations in Accra (Ghana)
AbstractObjectivesAreas with high HIV-incidence rates compared to the developed world may benefit from additional testing in blood banks and may show more favorable cost-effectiveness ratios. We evaluated the cost-effectiveness of adding p24 antigen, mini pool nucleic acid amplification testing (MP-NAT), or individual donation NAT (ID-NAT) to the HIV-antibody screening at the Korle Bu Teaching Hospital (Accra, Ghana), where currently only HIV-antibody screening is undertaken.MethodsThe residual risk of HIV transmission was derived from blood donations to the blood bank of the Korle Bu Teaching Hospital in 2004. Remaining life expectancies of patients receiving blood transfusion were estimated using the World Health Organization life expectancies. Cost-effectiveness ratios for adding the tests to HIV-antibody screening only were determined using a decision tree model and a Markov model for HIV.ResultsThe prevalence of HIV was estimated at 1.51% in 18,714 donations during 2004. The incremental cost per disability-adjusted life-year (DALY) averted was US3142 for MP-NAT and US73.85 and averting 0.86 DALY per transfused patient. Up to a willingness-to-pay of US11,828 per DALY averted, ID-NAT was significantly more cost-effective than the other strategies.ConclusionsAdding p24 antigen, MP-NAT, or ID-NAT to the current antibody screening cannot be regarded as a cost-effective health-care intervention for Ghana
From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Galaxies
Using V and I images obtained with WFPC2/HST, we investigate compact stellar
structures within tidal tails. Six regions of tidal debris in the four classic
``Toomre Sequence'' mergers: NGC 4038/39 (``Antennae''), NGC 3256, NGC 3921,
and NGC 7252 (``Atoms for Peace'') have been studied in order to explore how
the star formation depends upon the local and global physical conditions. These
mergers sample a range of stages in the evolutionary sequence and tails with
and without embedded tidal dwarf galaxies. The six tails are found to contain a
variety of stellar structures, with sizes ranging from those of globular
clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure
the luminosities and colors of the star clusters. NGC 3256 is found to have a
large population of blue clusters (0.2 < V-I < 0.9), particularly in its
Western tail, similar to those found in the inner region of the merger. In
contrast, NGC 4038/39 has no clusters in the observed region of the tail, only
less luminous point sources likely to be individual stars. A significant
cluster population is clearly associated with the prominent tidal dwarf
candidates in the eastern and western tails of NGC 7252. The cluster-rich
Western tail of NGC 3256 is not distinguished from the others by its dynamical
age or by its total HI mass. However, the mergers that have few clusters in the
tail all have tidal dwarf galaxies, while NGC 3256 does not have prominent
tidal dwarfs. We speculate that star formation in tidal tails may manifest
itself either in small structures like clusters along the tail or in large
structures such as dwarf galaxies, but not in both. Also, NGC 3256 has the
highest star formation rate of the four mergers studied, which may contribute
to the high number of star clusters in its tidal tails.Comment: Accepted to Astronomical Journal. 34 pages including 15 figures and 4
table
Exploring Neutral Hydrogen and Galaxy Evolution with the SKA
One of the key science drivers for the development of the SKA is to observe
the neutral hydrogen, HI, in galaxies as a means to probe galaxy evolution
across a range of environments over cosmic time. Over the past decade, much
progress has been made in theoretical simulations and observations of HI in
galaxies. However, recent HI surveys on both single dish radio telescopes and
interferometers, while providing detailed information on global HI properties,
the dark matter distribution in galaxies, as well as insight into the
relationship between star formation and the interstellar medium, have been
limited to the local universe. Ongoing and upcoming HI surveys on SKA
pathfinder instruments will extend these measurements beyond the local universe
to intermediate redshifts with long observing programmes. We present here an
overview of the HI science which will be possible with the increased
capabilities of the SKA and which will build upon the expected increase in
knowledge of HI in and around galaxies obtained with the SKA pathfinder
surveys. With the SKA1 the greatest improvement over our current measurements
is the capability to image galaxies at reasonable linear resolution and good
column density sensitivity to much higher redshifts (0.2 < z < 1.7). So one
will not only be able to increase the number of detections to study the
evolution of the HI mass function, but also have the sensitivity and resolution
to study inflows and outflows to and from galaxies and the kinematics of the
gas within and around galaxies as a function of environment and cosmic time out
to previously unexplored depths. The increased sensitivity of SKA2 will allow
us to image Milky Way-size galaxies out to redshifts of z=1 and will provide
the data required for a comprehensive picture of the HI content of galaxies
back to z~2 when the cosmic star formation rate density was at its peak.Comment: 25 pages, 5 figures, 3 tables. Contribution to the conference
'Advancing Astrophysics with the Square Kilometre Array', June 8-13, 2014,
Giardini Naxos, Ital
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