Growing in number, passive in nature: tracing the evolution of the most massive quiescent galaxies since z ~ 0.8 with BOSS and DESI

Abstract: Luminous Red Galaxies (LRGs) are among the most massive galaxies at any epoch, and lack ongoing star formation. As systems hosting most of the baryonic mass in the local Universe, they preserve imprints of the quenching mechanisms in the early Universe. We exploited the large BOSS and DESI spectroscopic datasets to perform the first homogeneous and continuous mapping of the evolution of stellar population properties of a complete sample of the most massive LRGs ($\log (M_*/\mathrm{M_\odot})> 11.5$) at 0.15 < z < 0.8. By consistently fitting the same spectral indices at all redshifts, we measured trends of [Fe/H], [alpha/Fe], and light-weighted age as a function of redshift. These galaxies exhibit a passive light-weighted age evolution and flat [Fe/H] and [alpha/Fe] trends towards lower redshift, indicating genuinely passive evolution. These trends are robust against the choice of stellar population models and analysis assumptions, and they support the predictions from IllustrisTNG, which predict negligible chemical evolution for the most massive quenched systems at z < 0.8. Our results suggest that, despite nearly 5 Gyr of cosmic time and a 3-4x increase in number density, the stellar population properties of massive quiescent galaxies remain essentially unchanged since z ~ 0.8. This shows a negligible progenitor bias below z ~ 0.8, and a genuinely passive evolution. Newly added systems after $z \sim 0.8$ were already largely quenched and chemically mature, while subsequent evolution was dominated by dry mergers without altering the bulk of the stellar populations.