Avoiding progenitor bias: The structural and mass evolution of Brightest Group and Cluster Galaxies in Hierarchical models since z~1

TL;DR

This study investigates the evolution of brightest group and cluster galaxies since z~1, addressing progenitor bias by tracking galaxy progenitors and comparing observed growth with hierarchical models.

Contribution

It introduces a method to mitigate progenitor bias by evolving galaxy host halo masses and matching descendants, and evaluates hierarchical models against observed galaxy growth.

Findings

Galaxies have increased in stellar mass by over a factor of 2 since z~1.

Effective radius has grown by more than a factor of 2.5 since z~1.

Hierarchical models with limited satellite stripping reproduce observed growth.

Abstract

The mass and structural evolution of massive galaxies is one of the hottest topics in galaxy formation. This is because it may reveal invaluable insights into the still debated evolutionary processes governing the growth and assembly of spheroids. However, direct comparison between models and observations is usually prevented by the so-called "progenitor bias", i.e., new galaxies entering the observational selection at later epochs, thus eluding a precise study of how pre-existing galaxies actually evolve in size. To limit this effect, we here gather data on high-redshift brightest group and cluster galaxies, evolve their (mean) host halo masses down to z=0 along their main progenitors, and assign as their "descendants" local SDSS central galaxies matched in host halo mass. At face value, the comparison between high redshift and local data suggests a noticeable increase in stellar mass of a factor of >2 since z~1, and of >2.5 in mean effective radius. We then compare the inferred stellar mass and size growth with those predicted by hierarchical models for central galaxies, selected at high redshifts to closely match the halo and stellar mass bins as in the data. Only hierarchical models characterized by very limited satellite stellar stripping and parabolic orbits are capable of broadly reproducing the stellar mass and size increase of a factor ~2-4 observed in cluster galaxies since z ~1. The predicted, average (major) merger rate since z~1 is in good agreement with the latest observational estimates.