Ultraviolet ISM Diagnostics for Star-Forming Galaxies I. Tracers of
  Metallicity and Extinction
  We have observed a sample of 14 nearby ($z \sim 0.03$) star-forming blue
compact galaxies in the rest-frame far-UV ($\sim1150-2200 \AA$) using the
Cosmic Origins Spectrograph on the Hubble Space Telescope. We have also
generated a grid of stellar population synthesis models using the Starburst99
evolutionary synthesis code, allowing us to compare observations and
theoretical predictions for the SiIV_1400 and CIV_1550 UV indices; both are
comprised of a blend of stellar wind and interstellar lines and have been
proposed as metallicity diagnostics in the UV. Our models and observations both
demonstrate that there is a positive linear correlation with metallicity for
both indices, and we find generally good agreement between our observations and
the predictions of the Starburst99 models. By combining the rest-frame UV
observations with pre-existing rest-frame optical spectrophotometry of our blue
compact galaxy sample, we also directly compare the predictions of metallicity
and extinction diagnostics across both wavelength regimes. This comparison
reveals a correlation between the UV absorption and optical strong-line
diagnostics, offering the first means of directly comparing ISM properties
determined across different rest-frame regimes. Finally, using our Starburst99
model grid we determine theoretical values for the short-wavelength UV
continuum slope, $\beta_{18}$, that can be used for determining extinction in
rest-frame UV spectra of star-forming galaxies. We consider the implications of
these results and discuss future work aimed at parameterizing these and other
environmental diagnostics in the UV as well as the development of robust
comparisons between ISM diagnostics across a broad wavelength baseline.
