Beyond halo mass: galactic conformity as a smoking gun of central galaxy assembly bias

Abstract

Quenched central galaxies tend to reside in a preferentially quenched\nlarge-scale environment, a phenomenon that has been dubbed galactic conformity.\nRemarkably, this tendency persists out to scales far larger than the virial\nradius of the halo hosting the central. Therefore, conformity manifestly\nviolates the widely adopted assumption that the dark matter halo mass Mvir\nexclusively governs galaxy occupation statistics. This paper is the first in a\nseries studying the implications of the observed conformity signal for the\ngalaxy-dark matter connection. We show that recent measurements of conformity\non scales R ~1-5 Mpc imply that central galaxy quenching statistics cannot be\ncorrectly predicted with the knowledge of Mvir alone. We also demonstrate that\nejected (or `backsplash') satellites cannot give rise to the signal. We then\ninvoke the age matching model, which is predicated on the co-evolution of\ngalaxies and halos. We find that this model produces a strong signal, and that\ncentral galaxies are solely responsible. We conclude that large-scale `2-halo'\nconformity represents a smoking gun of central galaxy assembly bias, and\nindicates that contemporary models of satellite quenching have systematically\nover-estimated the influence of post-infall processes.\n