Star formation and the extreme outer regions of disk galaxies

A knowledge of the physical processes which govern the conversion of interstellar gas into stars is of fundamental importance for models of galaxy formation and evolution. This thesis examines various aspects of the star formation process through a series of complementary, observational studies. A novel aspect of this work is the particular emphasis on the low gas surface density, optically-faint outer reaches of galactic disks, lying at or beyond the optical radius, R{dollar}sb{lcub}25{rcub}.{dollar}; Deep, wide-field H{dollar}alpha{dollar} is presented for a sample of 5 nearby, large spiral galaxies, leading to the discovery of numerous small, faint HII regions in the extreme outer disks {dollar}(sbsp{lcub}sim{rcub}{lcub}>{rcub}2 rm Rsb{lcub}25{rcub}){dollar} of 3 galaxies. The outer HII regions are observed to lie along organized spiral arm structures. Abrupt declines in the star formation rate per unit area are observed near R{dollar}sb{lcub}25{rcub}.{dollar}; Long-slit spectroscopy is presented for a sub-sample of our newly-discovered extreme outer disk HII regions. These data are used to measure the O/H and N/O abundances out to radii of 1.5-2R{dollar}sb{lcub}25{rcub}{dollar} in 3 galaxies. The outermost HII regions are found to have O/H abundances of 10-15% solar, N/O abundances in the range 20-25% solar and show evidence for diminished extinction.; Very deep narrow-band images are presented of 3 Sculptor Group spirals, revealing the existence of copious amounts of faint, diffuse ionized gas lying outside the boundaries of traditional HII regions. Study of the large-scale distribution, global energetics and ionization state of this component reveals an intimate connection with massive star formation; our results are consistent with the hypothesis that the DIG is ionized by Lyman-continuum photons which leak out of traditional HII regions.; Our new H{dollar}alpha{dollar} data are combined with measurements from the literature to test various models of large scale star formation. Neither pure nor modified Schmidt-laws with a dependence on gas surface density are able to explain the observations. The threshold model for star formation is shown to provide an adequate description of where star formation occurs, but not the rate. The observed rates are well-explained by assuming a pure Schmidt-law with a dependence on gas volume density, incorporating the effects of disk flaring.