Correlating galaxy colour and halo concentration: A tunable Halo Model of galactic conformity

TL;DR

This paper extends the Halo Occupation Distribution framework to model galactic conformity by correlating galaxy color with halo concentration, revealing scale-dependent effects and the role of assembly bias in galaxy evolution.

Contribution

It introduces a tunable HOD model that captures both 1-halo and 2-halo galactic conformity, linking galaxy color to halo properties and assembly bias effects.

Findings

Matching conformity at fixed halo mass with SDSS data

Conformity primarily a 1-halo effect at small scales

Genuine 2-halo conformity appears at large scales

Abstract

We extend the Halo Occupation Distribution (HOD) framework to generate mock galaxy catalogs exhibiting varying levels of "galactic conformity", which has emerged as a potentially powerful probe of environmental effects in galaxy evolution. Our model correlates galaxy colours in a group with the concentration of the common parent dark halo through a "group quenching efficiency" $\rho$ which makes older, more concentrated halos $\textit{at fixed mass}$ preferentially host redder galaxies. We find that, for a specific value of $\rho$, this 1-halo conformity matches corresponding measurements in a group catalog based on the Sloan Digital Sky Survey. Our mocks also display conformity at large separations from isolated objects, potentially an imprint of halo assembly bias. A detailed study - using mocks with assembly bias erased while keeping 1-halo conformity intact - reveals a rather nuanced situation, however. At separations $\lesssim 4$Mpc, conformity is mainly a 1-halo effect dominated by the largest halos and is $\textit{not}$ a robust indicator of assembly bias. Only at very large separations ($\gtrsim 8$Mpc) does genuine 2-halo conformity, driven by the assembly bias of small halos, manifest distinctly. We explain all these trends in standard Halo Model terms. Our model opens the door to parametrized HOD analyses that self-consistently account for galactic conformity at all scales.