The nebular properties of star-forming galaxies at intermediate redshifts from LEGA-C

Abstract

We present a detailed emission line study of the partial rest-optical (λobs ≈ 3600 − 5600 ˚A) nebular spectra of N ≈ 340 star-forming galaxies at 0.6 < z < 1.0 obtained with VLT/VIMOS as part of the Large Early Galaxy Census (LEGA-C). The LEGA-C survey is representative of star-forming galaxies at these redshifts with stellar masses log(M∗/M⊙) ∼ 10−11 and star formation rates log(SFR/[M⊙/yr]) ∼ 0−2. We compare this survey with observations of lowredshift (z ∼ 0) galaxies from the Sloan Digital Sky Survey (SDSS) and high-redshift (z ∼ 2) galaxies from the Keck Baryonic Structure Survey (KBSS). We find that 0.6 < z < 1.0 galaxies are more similar to z ∼ 0 galaxies than z ∼ 2 galaxies, while still showing somewhat different nebular ionization and excitation at low stellar masses (10.00 < log(M∗/M⊙) < 10.25) when compared to z ∼ 0 galaxies. We explore selection effects, contributions from active galactic nuclei, and variations in physical conditions (ionization parameter and oxygen abundance) as potential explanation for these differences, finding variations in physical conditions (higher nebular ionization and lower nebular enrichment) to be the most likely explanation. These results show that galaxies experience dramatic changes between z ∼ 2 and z ∼ 1, while apparently completing the majority of their assembly and evolution by 0.6 < z < 1.0. Our conclusions are supported by a subsample of N ≈ 20 LEGA-C galaxies with additional near-infrared spectra, allowing for complete rest-optical coverage. The tools we developed for this analysis will be used to interpret the abundance of complete rest-optical nebular spectra that will soon be available from the Subaru Prime Focus Spectrograph (Subaru/PFS) and the Very Large Telescope Multi-Object Optical and NIR Spectrograph (VLT/MOONS).