Day 1256

Monday.

1705.04327
The immitigable nature of assembly bias: the impact of halo definition on assembly bias
Villarreal, Zentner, et al

DM halo clustering depends not only on halo mass, but also on other properties such as concentration and shape.  This phenomenon is known broadly as assembly bias.  Explore the dependence of assembly bias on halo definition, parameterized by spherical overdensity parameter, Delta.  Summarize the strength of concentration-, shape- and spin-dependent halo clustering as a function of halo mass and halo definition.  Concentration-dependent clustering depends strongly on mass at all Delta.  For conventional halo definitions (Delta ~ 200m-600m), concentration-dependent clustering at low mass is driven by a population of haloes that is altered through interactions with neighboring haloes.  Concentraiton-dependent clustering can be greatly reduced through a mass-dependent halo definition with Delta ~ 20m-40m for haloes with M_200m<~1e12 Msun/h.  Smaller Delta implies larger radii and mitigates assembly bias at low mass by subsuming altered, so-called backsplash haloes into now larger host haloes.  At higher masses (M_200m.~1e13 Msun/h) larger over densities, Delta>~600m, are necessary.  Shape- and spin-dependent clustering are significant for all halo definitions that is explored and exhibit a relatively weaker mass dependence.  Generally, both the strength and the sense of assembly bias depend on halo definition, varying significantly even among common definitions.  Identify no halo definition that mitigates all manifestations of assembly bias.  A halo definition that mitigates assembly bias based on one halo property (e.g., concentration) must be mass dependent.  The halo definitions that best mitigate concentration-dependent halo clustering do not coincide with the expected average splash back radii at fixed halo mass.