LEGA-C stellar population scaling relations

I. Chemo-archaeological downsizing trends at z ∼ 0.7

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 Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁶ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK
⁷ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI, 48109, USA
⁸ Department of Astronomy and Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA, 16802, USA
⁹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA, 16802, USA
¹⁰ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA, 16802, USA
¹¹ Graduate Institute of Astrophysics, National Taiwan University, Taipei, 10617, Taiwan
¹² Department of Physics and Center for Theoretical Physics, National Taiwan University, Taipei, 10617, Taiwan
¹³ Physics Division, National Center for Theoretical Sciences, Taipei, 10617, Taiwan
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
¹⁵ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI, 53706, USA
¹⁶ Dipartimento di Fisica, Università di Trento, Via Sommarive 14, I-38123, Povo, (TN), Italy

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Received: 1 April 2025
Accepted: 18 January 2026

Abstract

We analysed the stellar population properties of a well-defined sample of 552 galaxies at redshift 0.6 < z < 0.77 drawn from the LEGA-C spectroscopic survey. This paper is the first of a series, and it is aimed at (i) presenting the catalogue of revised absorption indices for LEGA-C DR3 and of the inferred physical parameter estimates while describing their systematic uncertainties and at (ii) deriving benchmark scaling relations for the general massive galaxy population at intermediate redshift. We estimated light-weighted mean ages and stellar metallicities through careful analysis of key absorption features in the stellar continuum spectra of the galaxies coupled with photometry. The observables were interpreted in a Bayesian framework with a comprehensive library of model spectra based on stochastic star formation histories, chemical enrichment histories, and dust attenuations. We discuss various sources of systematic uncertainties within our method as well as systematic differences with results from other spectral fitting approaches. We derived volume-weighted scaling relations connecting light-weighted mean ages and stellar metallicities with galaxy stellar mass for the general galaxy population at ⟨z⟩ = 0.7 and masses > 10¹⁰ M_⊙. We find the downsizing trends observed in the local Universe to be already in place 6 Gyr ago. We also observe a bimodal distribution of light-weighted ages as a function of mass, transitioning around 10¹¹ M_⊙. Such a bimodality is not observed in the stellar metallicity-mass relation, which changes from a steep to a flat regime across M_* ∼ 10^10.8 M_⊙. Similar trends in age and metallicity also emerge as a function of velocity dispersion, but with a sharper transition from young to old around log σ_* = 2.3. Differences with respect to the trends as a function of stellar mass suggest that age is primarily dependent on velocity dispersion below and above the transition regime, while both the stellar mass and the depth of the total gravitational potential well (as traced by the velocity dispersion) contribute to stellar metallicity. We release the catalogues of revised absorption index measurements for LEGA-C DR3 used in this work and of the inferred stellar population physical parameters to public repositories.