Stellar populations of over one 1000 z ∼ 0.8 galaxies from LEGA-C: ages and star formation histories from D_n4000 and Hδ

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 4000Å break (D_n4000) and Balmer absorption line strengths (probed by Hδ) from 1019 high-quality spectra of z = 0.6−1.0 galaxies with M∗ = 2 x 10¹⁰M⊙−3×10¹¹M_⊙. Our analysis serves as a first illustration of the power of high-resolution, high-S/N continuum spectroscopy at intermediate redshifts as a qualitatively new tool to constrain galaxy formation models. The observed D_n4000-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 x 10¹⁰M⊙, we see a bimodal D_n4000-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 x 10¹¹M⊙ 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 odd with pure passive evolution given a difference in lookback time of > 5 Gyr; younger galaxies must grow to > 10¹¹M⊙ in the meantime, and/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_n4000, implying larger short-term variations in star-formation activity.