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.

Figures (12)

• Figure 1: Apparent angular size (in arcsec) of a typical massive quiescent galaxy with a physical effective diameter of $10$ kpc (R$_\text{eff} = 5$ kpc) as a function of redshift. The horizontal red (green) line indicates the BOSS (DESI) fibre aperture. The shaded red (green) area marks the redshift range considered for the BOSS (DESI) sample.
• Figure 2: Histogram of the stellar mass differences for the matched galaxies in DESI and BOSS at $0.15 < z < 0.4$. The dotted red line marks the median value of the distribution.
• Figure 3: SNR distributions for the DESI stacked spectra, in black, and BOSS, in red, in the rest-frame range $4000-4200\,\text{\AA}$.
• Figure 4: FIF application on the selected indices for a stacked spectrum in the DESI sample. The vertical dashed brown lines indicate the feature boundaries for each index while the grey shaded area represent the pseudo-continua regions used for normalisation. In the upper subplots, the black lines and blue shaded regions correspond to the stacked spectrum and its associated uncertainty, respectively. The solid red line represents the best-fit derived from the posterior distribution. The red lines in the lower subplots show the residuals between the observed spectrum and the best fit, with the blue shaded region indicating the relative uncertainties of the stacked spectrum.
• Figure 5: Example of the joint and marginal posterior distributions of light-weighted age, [M/H], and [$\alpha$/Fe] for a DESI stacked spectrum at $z = 0.4$ and $\log (M_*/\mathrm{M_\odot}) = 11.6$. Contours represent the $68\%$ and $95\%$ probability level. The $16\%$, $50\%$ and $84\%$ intervals are indicated as dashed lines.