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

Abstract

We present a detailed study of the partial rest-optical (λ obs ≈ 3600–5600 Å) 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 ∼ 0) galaxies from the Sloan Digital Sky Survey (SDSS), intermediate-redshift (z ∼ 1.6) galaxies from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS Survey, and high-redshift (z ∼ 2) galaxies from the Keck Baryonic Structure Survey (KBSS). At a look-back time of 6–8 Gyr, galaxies with stellar masses appear remarkably similar to z ∼ 0 galaxies in terms of their nebular excitation, as measured using [O iii] λ5008/Hβ. There is some evidence that 0.6 < z < 1.0 galaxies with lower M * have higher [O iii] λ5008/Hβ than z ∼ 0 galaxies and are more similar to less evolved z ∼ 1.6 and z ∼ 2 galaxies, which are offset from the z ∼ 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] λ5008/Hβ and find somewhat higher ionization in 0.6 < z < 1.0 galaxies with lower M * relative to z ∼ 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.