Inferring the star-formation histories of the most massive and passive early-type galaxies at z < 0.3

Context. In the Λ cold dark matter (ΛCDM) cosmological framework, massive galaxies are the end-points of the hierarchical evolution and are therefore key probes for understanding how the baryonic matter evolves within the dark matter halos. Aims. The aim of this work is to use the archaeological approach in order to infer the stellar population properties and star formation histories of the most massive (M > 1010.75 M⊙) and passive early-type galaxies (ETGs) at 0 < z < 0.3 (corresponding to a cosmic time interval of ~3.3 Gyr) based on stacked, high signal-to-noise (S/N), spectra extracted from the Sloan Digital Sky Survey (SDSS). Our study is focused on the most passive ETGs in order to avoid the contamination of galaxies with residual star formation activity and extract the evolutionary information on the oldest envelope of the global galaxy population. Methods. Unlike most previous studies in this field, we did not rely on individual absorption features such as the Lick indices, but we used the information present in the full spectrum with the STARLIGHT public code, adopting different stellar population synthesis models. Successful tests have been performed to assess the reliability of STARLIGHT to retrieve the evolutionary properties of the ETG stellar populations such as the age, metallicity and star formation history. The results indicate that these properties can be derived with accuracy better than 10% at S/N ≳ 10–20, and also that the procedure of stacking galaxy spectra does not introduce significant biases into their retrieval. Results. Based on our spectral analysis, we found that the ETGs of our sample are very old systems – the most massive ones are almost as old as the Universe. The stellar metallicities are slightly supersolar, with a mean of Z ~ 0.027 ± 0.002 and Z ~ 0.029 ± 0.0015 (depending on the spectral synthesis models used for the fit) and do not depend on redshift. Dust extinction is very low, with a mean of AV ~ 0.08 ± 0.030 mag and AV ~ 0.16 ± 0.048 mag. The ETGs show an anti-hierarchical evolution (downsizing) where more massive galaxies are older. The SFHs can be approximated with a parametric function of the form SFR(t) ∝ τ− (c + 1)tc exp(−t/τ), with typical short e-folding times of τ ~ 0.6−0.8 Gyr (with a dispersion of ±0.1 Gyr) and c ~ 0.1 (with a dispersion of ±0.05). Based on the reconstructed SFHs, most of the stellar mass (≳75%) was assembled by z ~ 5 and ≲4% of it can be ascribed to stellar populations younger than ~1 Gy