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Session 68 - Disk Galaxies.
Display session, Wednesday, January 15
Metropolitan Ballroom,
A knowledge of the chemical abundance of outer disk gas is of great importance for several problems in disk galaxy formation and evolution. The outer regions of present-day disks, characterized by low gas surface densities and high gas fraction, have physical conditions which may be similar in many ways to those existing during the early stages of galaxy evolution. To date, only a handful of outer disk HII regions have known chemical abundances and none of these lie at galactocentric radii beyond 1.1 R_25.
As part of a large project to study recent star formation in the outer disks of nearby galaxies, we have identified a number of galaxies with HII regions located at galactocentric radii in the range R_25 \le R \le 1.8 R_Holmberg. We present here preliminary results of a spectroscopic study to investigate the chemical abundances of these outermost HII regions. These data allow us to study the form of galactic abundance gradients beyond the canonical edge of the optical disk and we discuss the implications of our results for disk galaxy evolution. We compare our measured abundances to those observed in damped Lyman alpha systems observed in quasar absorption line spectra at high redshift.
