Stellar Populations of over 1000 z ∼ 0.8 Galaxies from LEGA-C: Ages and Star Formation Histories from Dn4000 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 4000 angstrom break (D(n)4000) and Balmer absorption line strengths (probed by H delta) from 1019 high-quality spectra of z = 0.6-1.0 galaxies with M-*. = 2 x 10(10)M(circle dot) to 3 x 10(11)M(circle dot). 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(n)4000-EW(H delta) distribution of our sample overlaps with the distribution traced by present-day galaxies, but z similar to 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 10(10)M(circle dot), we see a bimodal D(n)4000-EW(H delta) distribution at z similar to 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 10(11)M(circle dot) being almost all older than 2 Gyr. At the same time, we estimate that galaxies in this high-mass range are only similar to 3. Gyr younger than their z. similar to 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(11)M(circle dot) in the meantime, or small amounts of young stars must keep the lightweighted ages young. Star-forming galaxies at z similar to 0.8 have stronger Hd absorption than present-day galaxies with the same D(n)4000, implying larger short-term variations in star formation activity.