Re-assessing the stellar population scaling relations of the galaxies in the Local Universe

¹ Dipartimento di Fisica, Università di Trento, Via Sommarive 14, I-38123 Povo (TN), Italy
² INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
³ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, I-50019 Sesto Fiorentino, Italy

^⋆ Corresponding author: daniele.mattolini@unitn.it; daniele.mattolini@inaf.it

Received: 1 April 2025
Accepted: 13 August 2025

Abstract

Context. Local galaxies follow scaling relations between their mass and stellar population properties, such as age and metallicity. These relations encode fundamental information about the past evolutionary history of galaxies.

Aims. We want to revise stellar population scaling relations of local galaxies by leveraging the largest spectroscopic dataset provided by Data Release Seven of the Sloan Digital Sky Survey (SDSS DR7) at 0.005 < z < 0.22, using improved stellar population synthesis (SPS) models and novel SDSS aperture bias corrections.

Methods. We applied statistical weights to account for selection biases and implemented corrections to the SDSS fibre aperture limitations. Within a Bayesian framework, we estimated the stellar masses, mean stellar ages, and mean stellar metallicities by comparing the spectral indices and photometry with composite stellar population models. We adopted state-of-the-art ingredients and updated prescriptions to better capture the complexity of galaxies star formation and chemical enrichment histories. We also tested different models and priors.

Results. We estimated light-weighted mean stellar ages for 354 977 galaxies (S/N ≥ 10) and metallicities for 89 852 galaxies (S/N ≥ 20), and studied their dependence on stellar mass. Our key findings include: i) a revised bimodal distribution in the mass-age plane, with a young sequence (dominant at low masses) and an old sequence (dominant at high masses), partly overlapping in mass and transitioning at M_tr = 10^10.80 M_⊙; ii) a mass-metallicity relation (MZR) shifted ∼0.2 dex higher than in previous studies. Our aperture corrections produce mass-dependent reductions in masses, ages, and metallicities, enhancing the young sequence and steepening the MZR at low masses; iii) using Hα-based star formation rate classification, we found that while star-forming+young and quiescent+old correspondences generally hold, some exceptions exist for many galaxies. Quiescent galaxies show flatter, less scattered MZR than star-forming ones, converging at high masses; and, finally, iv) different SPS modelling assumptions significantly impact results, with star formation and chemical enrichment histories having the strongest effects.

Conclusions. These revised relations provide new benchmarks for galaxy evolution studies and simulations. Systematic effects of 0.1 − 0.2 dex can arise from uncorrected aperture biases and different SPS modelling choices. Consistent assumptions should be applied when comparing observations and models.