Correcting for the overabundance of low-mass quiescent galaxies in semi-analytic models

TL;DR

This study identifies and addresses the overabundance of low-mass quiescent galaxies in semi-analytic models by modifying orphan galaxy star-formation rates, resulting in better alignment with deep observational data across redshifts.

Contribution

The paper introduces a simple model to boost star formation in orphan galaxies, significantly improving the match between semi-analytic models and observations of low-mass passive galaxies.

Findings

Boosting orphan galaxy star formation reduces low-mass passive galaxy overabundance.

Higher resolution simulations alleviate but do not fully solve the orphan problem.

Separately analyzing orphan and resolved satellite galaxies is recommended.

Abstract

We compare the L-Galaxies semi-analytic model to deep observational data from the UKIDSS Ultra Deep Survey (UDS) across the redshift range $0.5 < z < 3$. We find that the over-abundance of low-mass, passive galaxies at high redshifts in the model can be attributed solely to the properties of `orphan' galaxies, i.e. satellite galaxies where the simulation has lost track of the host dark matter subhalo. We implement a simple model that boosts the star-formation rates in orphan galaxies by matching them to non-orphaned satellite galaxies at a similar evolutionary stage. This straightforward change largely addresses the discrepancy in the low-mass passive fraction across all redshifts. We find that the orphan problem is somewhat alleviated by higher resolution simulations, but the preservation of a larger gas reservoir in orphans is still required to produce a better fit to the observed space density of low-mass passive galaxies. Our findings are also robust to the precise definition of the passive galaxy population. In general, considering the vastly different prescriptions used for orphans in semi-analytic models, we recommend that they are analysed separately from the resolved satellite galaxy population, particularly with JWST observations reigniting interest in the low-mass regime in which they dominate.