Madore, B. F.Boissier, S.Gil de Paz, ArmandoNelson, E.Petrillo, K.2023-06-202023-06-202007Arp, H.C. 1966, Ap.J.Suppl., 14, 1 Arp, H.C. & Madore, B.F. 1987, A Catalogue of Southern Peculiar Galaxies and Associations, Cambridge University Press, Cambridge Boissier, S., et al. (2007) Ap.J. Suppl., in press (astro-ph 0609071) Donas, J., et al. (1981) A. & Ap., 97, L7 Fergusen, A., et al. 1998, Ap.J., 506, L19 Gil de Paz, A., et al. 2005, Ap.J., 627, L29 Karachentsev, I. 1988, AISAO, 26, 42 Kennicutt, R.C. 1998, Ap.J., 498, 181 Kravtsov, A.V., Gnedin, O.Y. & Klypin, A.A. 2004 Ap.J., 609, 482 Lynds, R., & Toomre, A. 1976 Ap.J., 209, 382 Martin, C.L., & Kennicutt, R.C. 2001, Ap.J., 555, 301 Theys, J.C. & Spiegel, E.A. 1976 Ap.J., 208, 650 Thilker, D.A., et al. 2005, Ap.J., 619, L79978-1-58381-309-6http://arxiv.org/abs/0801.1340https://hdl.handle.net/20.500.14352/53496© Astronomical Society of the Pacific. International Conference From Stars to Galaxies - Building the Pieces to Build up the Universe held in honor of Cesare Chiosis 65th Birthday (2006. Venice, Italy). We sincerely thank all of our colleagues on the GALEX Mission for their support in enabling the science being conducted by NASA’s Galaxy Evolution Explorer. Major portions of this research would not have been possible without the NASA/IPAC Extragalactic database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Significant support for this research was also provided by the Observatories of the Carnegie Institution of Washington.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 existence 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 Karachentsev's sample of more than 1,000 apparently/optically isolated galaxies.engbook partopen access52DiskAstrofísicaAstronomía (Física)