We report deep Subaru Halpha observations of the XUV disk of M83. These new observations enable the first complete census of very young stellar clusters over the entire XUV disk. Combining Subaru and GALEX data with a stellar population synthesis mod
el, we find that (1) the standard, but stochastically-sampled, initial mass function (IMF) is preferred over the truncated IMF, because there are low mass stellar clusters (10^{2-3}Msun) that host massive O-type stars; that (2) the standard Salpeter IMF and a simple aging effect explain the counts of FUV-bright and Halpha-bright clusters with masses >10^3Msun; and that (3) the Halpha to FUV flux ratio over the XUV disk supports the standard IMF. The Subaru Prime Focus Camera (Suprime-Cam) covers a large area even outside the XUV disk -- far beyond the detection limit of the HI gas. This enables us to statistically separate the stellar clusters in the disk from background contamination. The new data, model, and previous spectroscopic studies provide overall consistent results with respect to the internal dust extinction (Av~0.1 mag) and low metallicity (~0.2Zsun) using the dust extinction curve of SMC.
We present recent results from several on-going studies: The first addresses the question of gas-density thresholds for star formation, as probed by the outer disks of normal nearby galaxies. The second concerns the observational evidence for the exi
stence of gravitating non-luminous (GNL) galaxies, as predicted by most recent simulations of galaxy formation in Lambda-CDM cosmologies. We find that (1) If star formation is traced by far-ultraviolet light, then there is no evidence for a threshold to star formation at any gas density so far probed, and (2) there is no evidence for GNL galaxies gravitationally interacting with known optical systems based on the observations (a) that there are no ring galaxies without plausible optically visible intruders, (b) all peculiar galaxies in the Arp Atlas that are bodily distorted have nearby plausibly interacting companions, and (c) there are no convincingly distorted/peculiar galaxies within Karachentsevs sample of more than 1,000 apparently/optically isolated galaxies.
