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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 Karachentsevs sample of more than 1,000 apparently/optically isolated galaxies.
Tadpole Galaxies look like a star forming head with a tail structure to the side. They are also named cometaries. In a series of recent works we have discovered a number of issues that lead us to consider them extremely interesting targets. First, fr
The study of satellite galaxies can provide information on the merging and aggregation processes which, according to the hierarchical clustering models, form the larger spiral galaxies we observe. With the aim of testing hierarchical models of galaxy
We present narrow-band observations of the H$alpha$ emission in a sample of 31 satellite orbiting isolated giant spiral galaxies. The sample studied spans the range $-19<M_B <-15$ mag. The H$alpha$ emission was detected in all the spiral and irregula
We demonstrate spatially-resolved measurements of spontaneous and stimulated electron-photon interactions in nanoscale optical near fields using electron energy-loss spectroscopy (EELS), cathodoluminescence spectroscopy (CL), and photon-induced near-
The main observational results from radio continuum and polarization observations about the magnetic field strength and large-scale pattern for face-on and edge-on spiral galaxies are summarized and compared within our sample of galaxies of different