Overabundance of orphan galaxies in the UniverseMachine empirical galaxy formation model

Abstract

ABSTRACT Orphan galaxies that have lost a large fraction of the dark matter subhaloes have often been invoked in semi-analytical as well as empirical models of galaxy formation. However, observations have not yet constrained the distribution of orphan galaxies in such models so far. We run a mock cluster finder that mimics the optical cluster finding technique of the redMaPPer algorithm on a catalogue of galaxies with quenched star formation from one such empirical model, the UniverseMachine, and obtain the prevalence of orphan galaxies in these clusters as a function of their cluster-centric distance. We compare the fraction of orphan galaxies with the upper limits derived based on our prior observations of the weak lensing signals around satellite galaxies from SDSS redMaPPer clusters. Although the orphan fraction from the UniverseMachine is marginally consistent with the upper limits in the innermost regions of galaxy clusters spanning $[0.1, 0.3]\, {h^{-1}\,\rm Mpc}$, we observe that the orphan fractions substantially violate the upper limits in the outer regions of galaxy clusters beyond $0.3\,{h^{-1}\,\rm Mpc}$. We discuss the reasons, plausible improvements to the model, and how observations can be used to constrain such models further.