Correlating galaxy colour and halo concentration: a tunable halo model of galactic conformity

Abstract

We extend the Halo Occupation Distribution (HOD) framework to generate mock\ngalaxy catalogs exhibiting varying levels of "galactic conformity", which has\nemerged as a potentially powerful probe of environmental effects in galaxy\nevolution. Our model correlates galaxy colours in a group with the\nconcentration of the common parent dark halo through a "group quenching\nefficiency" $\\rho$ which makes older, more concentrated halos $\\textit{at fixed\nmass}$ preferentially host redder galaxies. We find that, for a specific value\nof $\\rho$, this 1-halo conformity matches corresponding measurements in a group\ncatalog based on the Sloan Digital Sky Survey. Our mocks also display\nconformity at large separations from isolated objects, potentially an imprint\nof halo assembly bias. A detailed study - using mocks with assembly bias erased\nwhile keeping 1-halo conformity intact - reveals a rather nuanced situation,\nhowever. At separations $\\lesssim 4$Mpc, conformity is mainly a 1-halo effect\ndominated by the largest halos and is $\\textit{not}$ a robust indicator of\nassembly bias. Only at very large separations ($\\gtrsim 8$Mpc) does genuine\n2-halo conformity, driven by the assembly bias of small halos, manifest\ndistinctly. We explain all these trends in standard Halo Model terms. Our model\nopens the door to parametrized HOD analyses that self-consistently account for\ngalactic conformity at all scales.\n