LEGA-C stellar population scaling relations

II. Dissecting mass-complete archaeological trends and their evolution since z ∼ 0.7 with LEGA-C and SDSS

¹ INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50126, Firenze, Italy
² Sterrenkundig Observatorium Universiteit Gent, Krijgslaan 281 S9, B-9000, Gent, Belgium
³ STAR Institute, Université de Liège, Quartier Agora, Allée du six Aout 19c, B-4000, Liege, Belgium
⁴ Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA, 15260, USA
⁵ Dipartimento di Fisica, Università di Trento, Via Sommarive 14, I-38123, Povo, (TN), Italy
⁶ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI, 48109, USA
⁷ Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA, 16802, USA
⁸ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA, 16802, USA
⁹ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA, 16802, USA
¹⁰ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
¹¹ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK
¹² Graduate Institute of Astrophysics, National Taiwan University, Taipei, 10617, Taiwan
¹³ Department of Physics and Center for Theoretical Physics, National Taiwan University, Taipei, 10617, Taiwan
¹⁴ Physics Division, National Center for Theoretical Sciences, Taipei, 10617, Taiwan
¹⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
¹⁶ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI, 53706, USA

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Received: 1 April 2025
Accepted: 10 November 2025

Abstract

We analysed a sample of 552 galaxies from the LEGA-C spectroscopic survey (0.6 < z < 0.77), for which we estimated the stellar population parameters by a Bayesian analysis of the stellar absorption features and photometry. We investigated the effect of the current star formation activity on light-weighted mean stellar ages and metallicities and their median trends with stellar mass or velocity dispersion. The bimodality in the global age–mass relation stems from the different age distributions in the quiescent and star-forming populations. No bimodality is observed in the stellar metallicity-mass relation, although quiescent and star-forming galaxies have different distributions in this parameter space. We identified a high-metallicity sequence populated by quiescent and weakly star-forming galaxies. At masses lower than 10^10.8 M_⊙, the median stellar metallicity–mass relation of star-forming galaxies steepens as a consequence of the increasing scatter towards lower stellar metallicities for galaxies with an increasing specific star formation rate at fixed mass. Relying on a consistent analysis of SDSS DR7 spectra and accounting for aperture corrections, we quantified the evolution of the volume-weighted stellar age and stellar metallicity scaling relations between z = 0.7 and the present. We found negligible evolution in the stellar metallicity–mass relation of quiescent galaxies and for M_* > 10¹¹ M_⊙ galaxies in general. Lower-mass star-forming galaxies instead have typically lower metallicities than their local counterparts, indicating significant enrichment since z ∼ 0.7 in the low-mass regime. Notably, the median of the stellar ages of the general population and of quiescent galaxies has changed by only 2 Gyr between z = 0.7 and z = 0.1, which is less than expected from cosmic ageing. Some quiescent galaxies must evolve passively to reach the old boundary of the local population. In order to explain the evolution of the median trends, however, both individual evolution through rejuvenation and/or minor merging that affects the outer galaxy regions and population evolution through quenching of massive metal-rich star-forming galaxies are required.