Stellar and nebular diagnostics in the UV for star-forming galaxies

TL;DR

This paper evaluates the potential of UV stellar absorption and emission lines as diagnostics for metallicity and ionization in star-forming galaxies, extending models and proposing new diagnostic tools for upcoming JWST observations.

Contribution

It introduces extended UV emission models, identifies key UV line diagnostics, and develops new UV diagnostic diagrams for galaxy analysis.

Findings

UV emission lines can effectively constrain metallicity and ionization.

Certain UV line ratios are sensitive to nebular and stellar contributions.

Proposed UV BPT diagrams are suitable for JWST spectroscopy.

Abstract

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 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 CloudyFSPS nebular emission model of Byler et al. 2017, 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 SFRs. We identify combinations of strong emission lines that constrain metallicity and ionization parameter, including [CIII] 1907, CIII] 1909, OIII] 1661,1666, SiIII]1883,1892, CIV 1548,1551, NII] 1750,1752, and MgII 2796, and develop UV versions of the canonical "BPT" diagram. We quantify the relative contribution from stellar wind emission and nebular line emission to diagnostic line ratios that include the CIV 1548,1551 lines, and 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 Telescope.