Far-Ultraviolet Spectroscopy of Star-Forming Regions in Nearby Galaxies: Stellar Populations and Abundance Indicators

Abstract

We present FUSE spectroscopy and supporting data for star-forming regions in nearby galaxies, to examine their massive-star content and explore the use of abundance and population indicators in this spectral range for high-redshift galaxies. New far-ultraviolet spectra are shown for four bright H II regions in M33 (NGC 588, 592, 595, and 604), the H II region NGC 5461 in M101, and the starburst nucleus of NGC 7714, supplemented by the very-low-metallicity galaxy I Zw 18. In each case, we see strong Milky Way absorption systems from H2, but intrinsic absorption within each galaxy is weak or undetectable, perhaps because of the "UV bias" in which reddened stars which lie behind molecular-rich areas are also heavily reddened. We see striking changes in the stellar-wind lines from these populations with metallicity, suggesting that C II, C III, C IV, N II, N III, and P V lines are potential tracers of stellar metallicity in star-forming galaxies. Three of these relations - involving N IV, C III, and P V - are nearly linear over the range from O/H=0.05--0.8 solar. The major difference in continuum shapes among these systems is that the giant H II complex NGC 604 has a stronger continuum shortward of 950 A than any other object in this sample. Small-number statistics would likely go in the other direction; we favor this as the result of a discrete star-forming event ~3 Myr ago, as suggested by previous studies of its stellar population. (Supported by NASA grant NAG5-8959)