Setting firmer constraints on the evolution of the most massive, central galaxies from their local abundances and ages

TL;DR

This study constrains the formation and growth of the universe's most massive central galaxies by combining observational data on their abundance, ages, and dark matter halo histories, challenging traditional in-situ formation models.

Contribution

It provides new constraints on the assembly history of massive galaxies, suggesting they may form most of their stars early and rapidly, with limited subsequent size growth.

Findings

Massive galaxies (>3x10^11 Msun) predominantly reside in large clusters.

Spectral analysis indicates ages >10 Gyrs, formation redshift >2.

Stellar mass at formation rivals total baryonic mass in host haloes.

Abstract

There is still much debate surrounding how the most massive, central galaxies in the local universe have assembled their stellar mass, especially the relative roles of in-situ growth versus later accretion via mergers. In this paper, we set firmer constraints on the evolutionary pathways of the most massive central galaxies by making use of empirical estimates on their abundances and stellar ages. The most recent abundance matching and direct measurements strongly favour that a substantial fraction of massive galaxies with Mstar>3x10^11 Msun reside at the centre of clusters with mass Mhalo>3x10^13 Msun. Spectral analysis supports ages >10 Gyrs, corresponding to a formation redshift z_form >2. We combine these two pieces of observationally-based evidence with the mass accretion history of their host dark matter haloes. We find that in these massive haloes, the stellar mass locked up in the central galaxy is comparable to, if not greater than, the total baryonic mass at z_form. These findings indicate that either only a relatively minor fraction of their present-day stellar mass was formed in-situ at z_form, or that these massive, central galaxies form in the extreme scenario where almost all of the baryons in the progenitor halo are converted into stars. Interestingly, the latter scenario would not allow for any substantial size growth since the galaxy's formation epoch either via mergers or expansion. We show our results hold irrespective of systematic uncertainties in stellar mass, abundances, galaxy merger rates, stellar initial mass function, star formation rate and dark matter accretion histories.