The Nebular Properties of Star-forming Galaxies at Intermediate Redshift from the Large Early Galaxy Astrophysics Census

We present a detailed study of the partial rest-optical (lambda(obs) approximate to 3600-5600 angstrom) spectra of N =767 star-forming galaxies at 0.6 < z < 1.0 from the Large Early Galaxy Astrophysics Census (LEGA-C). We compare this sample with low-redshift (z similar to 0) galaxies from the Sloan Digital Sky Survey (SDSS), intermediate-redshift (z similar to 1.6) galaxies from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS Survey, and high-redshift (z similar to 2) galaxies from the Keck Baryonic Structure Survey (KBSS). At a look-back time of 6-8 Gyr, galaxies with stellar masses log(M-*/M-circle dot) > 10.50 appear remarkably similar to z similar to 0 galaxies in terms of their nebular excitation, as measured using [O III] lambda 5008/H beta. There is some evidence that 0.6 < z < 1.0 galaxies with lower M-* have higher [O III] lambda 5008/H beta than z similar to 0 galaxies and are more similar to less evolved z similar to 1.6 and z similar to 2 galaxies, which are offset from the z similar to 0 locus at all M-*. We explore the impact of selection effects, contributions from active galactic nuclei, and variations in physical conditions (ionization parameter and gas-phase oxygen abundance) on the apparent distribution of [O III] lambda 5008/H beta and find somewhat higher ionization in 0.6 < z < 1.0 galaxies with lower M-* relative to z similar to 0 galaxies. We use new near-infrared spectroscopic observations of a subsample of LEGA-C galaxies to investigate other probes of enrichment and excitation. Our analysis demonstrates the importance of obtaining complete rest-optical spectra of galaxies in order to disentangle these effects.