2,394 research outputs found
The Long-term Evolution of the Galactic Disk Traced by Dissolving Star Clusters
The Galactic disk retains a vast amount of information about how it came to
be, and how it evolved over cosmic time. However, we know very little about the
secular processes associated with disk evolution. One major uncertainty is the
extent to which stars migrate radially through the disk, thereby washing out
signatures of their past (e.g. birth sites). Recent theoretical work finds that
such "blurring" of the disk can be important if spiral arms are transient
phenomena. Here we describe an experiment to determine the importance of
diffusion from the Solar circle with cosmic time. Consider a star cluster that
has been placed into a differentially rotating, stellar fluid. We show that all
clusters up to ~10^4 solar masses, and a significant fraction of those up to
~10^5 solar masses, are expected to be chemically homogeneous, and that
clusters of this size can be assigned a unique "chemical tag" by measuring the
abundances of <~10 independent element groups, with better age and orbit
determinations allowing fewer abundance measurements. The star cluster
therefore acts like a "tracer dye", and the present-day distribution of its
stars provides a strong constraint on the rate of radial diffusion or migration
in the Galactic disk. Sellwood & Binney have argued for strong radial transport
driven by transient spiral perturbations: in principle, we could measure the
strength of this migration directly.Comment: ApJ, in press; 15 pages, 9 figures (ApJ format
The Chemical Signature of a Relic Star Cluster in the Sextans Dwarf Spheroidal Galaxy - Implications for Near-Field Cosmology
We present tentative evidence for the existence of a dissolved star cluster
at [Fe/H]=-2.7 in the Sextans dwarf spheroidal galaxy. We use the technique of
chemical tagging to identify stars that are highly clustered in a
multi-dimensional chemical abundance space (C-space). In a sample of six stars,
three, possibly four stars are identified as potential cluster stars. The
initial stellar mass of the parent cluster is estimated from two independent
observations to M*,init=1.9^{+1.5}_{-0.9} (1.6^{+1.2}_{-0.8}) x 10^5 M_sol,
assuming a Salpeter (Kroupa) initial mass function (IMF). If corroborated by
follow-up spectroscopy, this star cluster is the most metal-poor system
identified to date. Chemical signatures of remnant clusters in dwarf galaxies
like Sextans provide us with a very powerful probe to the high-redshift
Universe. From available observational data, we argue that the average star
cluster mass in the majority of the newly discovered ultra-faint dwarf galaxies
was notably lower than it is in the Galaxy today and possibly lower than in the
more luminous, classical dwarf spheroidal galaxies. Furthermore, the mean
cumulative metallicity function of the dwarf spheroidals falls below that of
the ultra-faints, which increases with increasing metallicity as predicted from
our stochastic chemical evolution model. These two findings, together with a
possible difference in the ratio suggest that the ultra-faint dwarf
galaxy population, or a significant fraction thereof, and the dwarf spheroidal
population, were formed in different environments and would thus be distinct in
origin.Comment: 18 pages, 8 figures, new Fig. 5, some clarifications, references
added, accepted for publication in the Ap
NASA/NREN: Next Generation Internet (NGI) Activities
Various issues associated with next generation internet (NGI) and the NREN (NASA Research and Education Network) activities are presented in viewgraph form. Specific topics include: 1) NREN architecture; 2) NREN applications; and 3) NREN applied research
Breach of Posterior Wall of Frontal Sinus: Management with Preservation of the Sinus
ObjectiveTo analyze outcomes after the management of mild (<1 mm) and moderately severe (>1 mm and <5 mm) breaches of the posterior wall of the frontal sinus with a goal of maintaining or restoring the functional status of the sinus.MethodsA retrospective analysis of prospectively accrued data was performed on patients with mild and moderately severe breaches of the posterior wall of their frontal sinus who were managed with the intent to preserve the frontal sinus. Data on presenting features, pathology, details on breaches of the posterior wall, management, outcome, and complications were collected from medical records and neuroimages.ResultsForty-two cases met inclusion criteria. Diagnostic categories included trauma in 34 cases, infection in 3, and other categories in another 5 cases. Five presented with cerebrospinal fluid rhinorrhea, and 26 had radiographic evidence of obstruction of a nasofrontal duct at time of presentation. Fifteen patients were managed without surgical intervention, and 27 underwent surgery. No complications occurred in the patients managed without surgery and 4 postoperative cerebrospinal leaks that were managed successfully with a period of drainage occurred in the surgical group. No patient developed meningitis or mucocele.ConclusionsMany patients with mild to moderately severe breaches of the posterior wall of the frontal sinus can be managed safely and effectively by techniques that preserve the anatomy and function of the frontal sinus
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
An HI census of Loose Groups of Galaxies
We present results from our Parkes Multibeam HI survey of 3 loose groups of
galaxies that are analogous to the Local Group. This is a survey of groups
containing only spiral galaxies with mean separations of a few hundred kpc, and
total areas of approximately 1 sq. Mpc; groups similar to our own Local Group.
We present a census of the HI-rich objects in these groups down to an M(HI),
1-sigma sensitivity ~7x10^5 M(sun), as well as the detailed properties of these
detections from follow-up Compact Array observations. We found 7 new HI-rich
members in the 3 groups, all of which have stellar counterparts and are,
therefore, typical dwarf galaxies. The ratio of low-mass to high-mass gas-rich
galaxies in these groups is less than in the Local Group meaning that the
``missing satellite'' problem is not unique. No high-velocity cloud analogs
were found in any of the groups. If HVCs in these groups are the same as in the
Local Group, this implies that HVCs must be located within ~300-400 kpc of the
Milky Way.Comment: 6 pages, to appear in the ASP proceedings of IAU Symposium 217,
"Recycling intergalactic and interstellar matter", eds. Pierre-Alain Duc,
Jonathan Braine, Elias Brink
Discovery of Numerous Dwarf Galaxies in the Two Nearest Groups of Galaxies
We report the discovery of a large population of dwarf irregular galaxies in
the two nearest groups of galaxies outside the Local Group, the Sculptor and
the Centaurus A groups (2.5 and 3.5 Mpc). Total areas of approximately 940 and
910 square degrees in these groups were scanned visually on the SRC J films to
find dwarf candidates. Redshifts were obtained by an HI survey carried out at
Parkes, with detection limits of 4 \times 10^6 \Msun and 7.8 \times 10^6 \Msun
, and 33 dwarf galaxies were successfully detected in the groups. A follow-up
optical survey (Halpha spectroscopy) detected a few more, and confirmed most of
the HI redshifts. A total of 16 and 20 dwarf galaxies are found in Sculptor and
Centaurus A, of which 6 are newly identified objects, and 5 more have newly
determined redshifts. In both groups the dwarf members show a wider spatial and
velocity distribution than the brighter members. From their radial velocities
and projected distances we estimate the crossing times of the groups, which
confirm that they are not yet virialised.Comment: 35 pages Latex (aaspp4.sty) including all tables and 7 postscript
figures. Figs 1,2,3 available at ftp://ftp.hq.eso.org/incoming/scote
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