PO-FENG WUWel A.V.DGallazzi ABezanson RPacifici CStraatman CFranx MBarišić IBell E.FBrammer G.BCalhau JChauke PHoudt J.VMaseda M.VMuzzin ARix H.-WSobral DSpilker JSande J.V.DDokkum P.VWild V.2022-11-112022-11-1120180004637Xhttps://www.scopus.com/inward/record.uri?eid=2-s2.0-85044045343&doi=10.3847%2f1538-4357%2faab0a6&partnerID=40&md5=a8d6dbb8545002731ea9ed478c3213fahttps://scholars.lib.ntu.edu.tw/handle/123456789/624931Drawing 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 break (Dn4000) and Balmer absorption line strengths (probed by Hδ) from 1019 high-quality spectra of z = 0.6-1.0 galaxies with M ∗ = 2 ×1010 M o to 3 ×1011 M o. 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 Dn4000-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 ×1010 M o, we see a bimodal Dn4000-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 ×1011 M o 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 >1011 M o 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 Dn4000, implying larger short-term variations in star formation activity. © 2018. The American Astronomical Society. All rights reserved..galaxies: evolution; galaxies: high-redshift; galaxies: stellar contentjournal article10.3847/1538-4357/aab0a62-s2.0-85044045343