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 Submitted to A