Far-ultraviolet spectroscopy of star-forming regions in nearby galaxies: Stellar populations and abundance indicators
We present Far Ultraviolet Spectroscopic Explorer 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 H-2, but intrinsic absorption within each galaxy is weak or undetectable, perhaps because of the "UV bias'' in which reddened stars that 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 Angstrom 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 approximate to 3 Myr ago, as suggested by previous studies of its stellar population.