Stellar and Nebular Diagnostics in the Ultraviolet for Star-forming Galaxies

There is a long history of using optical emission and absorption lines to constrain the metallicity and ionization parameters of gas in galaxies. However, comparable diagnostics are less well developed for the ultraviolet (UV). Here, we assess the diagnostic potential of both absorption and emission features in the UV and evaluate the diagnostics against observations of local and high-redshift galaxies. We use the Flexible Stellar Population Synthesis (FSPS) nebular emission model of Byler et al., extended to include emission predictions in the UV, to evaluate the metallicity sensitivity of established UV stellar absorption indices and to identify those that include a significant contribution from nebular emission. We present model UV emission-line fluxes as a function of metallicity and ionization parameter, assuming both instantaneous bursts and constant star formation rates. We identify combinations of strong emission lines that constrain metallicity and ionization parameters, including [C III] λ1907, C III] λ1909, O III] λ1661,1666, Si III] λ1883,1892, C IV λ1548,1551, N II] λ1750,1752, and Mg II λ2796, and we develop UV versions of the canonical “Baldwin Phillips Terlevich” diagram. We quantify the relative contribution from stellar wind emission and nebular line emission to diagnostic line ratios that include the C IV λ1548,1551 lines, and we also develop an observationally motivated relationship for N and C enrichment that improves the performance of photoionization models. We summarize the best diagnostic choices and the associated redshift range for low-, mid-, and high-resolution rest-UV spectroscopy in preparation for the launch of the James Webb Space T elescope.