Clues to the nature of ultra diffuse galaxies from estimated galaxy velocity dispersions

We describe how to estimate the velocity dispersions of ultra diffuse galaxies, UDGs, using a previously defined galaxy scaling relationship. The method is accurate for the two UDGs with spectroscopically measured dispersions, as well as for ultra compact galaxies, ultra faint galaxies, and stellar systems with little or no dark matter. This universality means that the relationship can be applied without further knowledge or prejudice regarding the structure of a galaxy. We then estimate the velocity dispersions of UDGs drawn from two published samples and examine the distribution of total masses. We find, in agreement with the previous studies of two individual UDGs, that these systems are dark matter dominated systems, and that they span a range of at least $10^{10} < M_{200}/M_\odot < 10^{12}$. These galaxies are not, as an entire class, either all dwarfs or all failed $L_*$ galaxies. Estimates of the velocity dispersions can also help identify interesting subsets of UDGs, such as those that are likely to have the largest mass-to-light ratios, for subsequent spectroscopic study.Comment: 4 pages, accepted for publication in MNRAS Letters (typographical error in Equation 1 corrected

The Star Clusters of the Large Magellanic Cloud: Structural Parameters

We present and analyze the radial luminosity profiles of a sample of 1066 stellar clusters in the Large Magellanic Cloud. By design, this study closely follows the compilation by Hill & Zaritsky of the structural parameters of stellar clusters in the Small Magellanic Cloud. Both King and Elson-Fall-Freeman (EFF) model profiles are fit to V-band surface brightness profiles measured from the Magellanic Cloud Photometric Survey images. We tabulate the concentration, central surface brightness, tidal radii, 90% enclosed luminosity radii (r_{90}), and local background luminosity density. Over two thirds of the clusters in the sample are adequately fit by one or both of these models. One notable and systematic exception, as in the SMC, are those clusters that lack a central brightness concentration, the "ring" clusters. While the bulk properties of the clusters are similar between the LMC and SMC populations, we find that the LMC lacks clusters that are as large, either in terms of core radii or r_{90}, as the largest in the SMC, perhaps a signature of larger tidal stresses in the LMC.Comment: Accepted for publication in AJ. 11 pages. Full version of Tables and Figures will be available through the Journa

On The Extended Knotted Disks of Galaxies

The stellar disks of many spiral galaxies are twice as large as generally thought. We use archival data from the Galaxy Evolution Explorer mission (GALEX) to quantify the statistical properties of young stellar clusters in the outer, extended disks of a sample of eleven nearby galaxies. We find an excess of sources between 1.25 and 2 optical radii, R(25), for five of the galaxies, which statistically implies that at least a quarter of such galaxies have this cluster population (90% confidence level), and no significant statistical excess in the sample as a whole beyond 2 optical radii, even though one galaxy (M 83) individually shows such an excess. Although the excess is typically most pronounced for blue (FUV -NUV < 1, NUV < 25) sources, there is also an excess of sources with redder colors. Although from galaxy to galaxy the number of sources varies significantly, on average, the galaxies with such sources have 75 +- 10 blue sources at radii between 1.25 and 2 R(25). In addition, the radial distribution is consistent with the extended dust emission observed in the far IR and with the properties of H-alpha sources, assuming a constant cluster formation rate over the last few hundred Myrs. All of these results suggest that the phenomenon of low-level star formation well outside the apparent optical edges of disks (R ~ R(25)) is common and long-lasting.Comment: 8 pages, accepted for publication in A