Stellar Populations of over 1000 z ̃ 0.8 Galaxies from LEGA-C: Ages and Star Formation Histories from D n 4000 and Hδ

Abstract

Drawing from the LEGA-C data set, we present the spectroscopic view of the stellar population across a large volume- and mass-selected sample of galaxies at large look-back time. We measure the 4000 Å break (D<SUB> n </SUB>4000) and Balmer absorption line strengths (probed by Hδ) from 1019 high-quality spectra of z = 0.6-1.0 galaxies with M <SUB>*</SUB> = 2 × 10<SUP>10</SUP> M <SUB>☉</SUB> to 3 × 10<SUP>11</SUP> M <SUB>☉</SUB>. Our analysis serves as a first illustration of the power of high-resolution, high signal-to-noise ratio continuum spectroscopy at intermediate redshifts as a qualitatively new tool to constrain galaxy formation models. The observed D<SUB> n </SUB>4000-EW(Hδ) distribution of our sample overlaps with the distribution traced by present-day galaxies, but z ̃ 0.8 galaxies populate that locus in a fundamentally different manner. While old galaxies dominate the present-day population at all stellar masses >2 × 10<SUP>10</SUP> M <SUB>☉</SUB>, we see a bimodal D<SUB> n </SUB>4000-EW(Hδ) distribution at z ̃ 0.8, implying a bimodal light-weighted age distribution. The light-weighted age depends strongly on stellar mass, with the most massive galaxies >1 × 10<SUP>11</SUP> M <SUB>☉</SUB> being almost all older than 2 Gyr. At the same time, we estimate that galaxies in this high-mass range are only ̃3 Gyr younger than their z ̃ 0.1 counterparts, at odds with purely passive evolution given a difference in look-back time of >5 Gyr; younger galaxies must grow to >10<SUP>11</SUP> M <SUB>☉</SUB> in the meantime, or small amounts of young stars must keep the light-weighted ages young. Star-forming galaxies at z ̃ 0.8 have stronger Hδ absorption than present-day galaxies with the same D<SUB> n </SUB>4000, implying larger short-term variations in star formation activity.