A large difference in the progenitor masses of active and passive galaxies in the EAGLE simulation

TL;DR

This study uses the EAGLE simulation to examine galaxy progenitor mass histories, revealing significant differences between active and passive galaxies that impact the accuracy of the cumulative number density matching method.

Contribution

The paper demonstrates that the cumulative number density method must be applied separately to active and passive galaxies due to their distinct mass growth histories, challenging previous assumptions.

Findings

Passive and active galaxies have different median mass histories.

The difference persists even for central galaxies.

Main progenitors at z=2 span two orders of magnitude in stellar mass.

Abstract

Cumulative number density matching of galaxies is a method to observationally connect descendent galaxies to their typical main progenitors at higher redshifts and thereby to assess the evolution of galaxy properties. The accuracy of this method is limited due to galaxy merging and scatter in the stellar mass growth history of individual galaxies. Behroozi et al. (2013) have introduced a refinement of the method, based on abundance matching of observed galaxies to the Bolshoi dark-matter-only simulation. The EAGLE cosmological hydro-simulation is well suited to test this method, because it reproduces the observed evolution of the galaxy stellar mass function and the passive fraction. We find agreement with the Behroozi et al. (2013) method for the complete sample of main progenitors of z = 0 galaxies, but we also find a strong dependence on the current star formation rate. Passive galaxies with a stellar mass up to 10^10.75 Msun have a completely different median mass history than active galaxies of the same mass. This difference persists if we only select central galaxies. This means that the cumulative number density method should be applied separately to active and passive galaxies. Even then, the typical main progenitor of a z = 0 galaxy already spans two orders of magnitude in stellar mass at z = 2.