Responses of Halo Occupation Distributions: a new ingredient in the halo model and the impact on galaxy bias

When: Wednesday, March 23, 2022, 2:30 PM - 3:30 PM
Where: Mile end campus

Speaker: Rodrigo Voivodic, DIPC (Donostia International Physics Center)

Halo occupation distribution (HOD) models describe the number of galaxies that reside in different haloes, and are widely used in galaxy-halo connection studies using the halo model (HM). Here, we introduce and study HOD response functions RgO that describe the response of the HODs to long-wavelength perturbations O. The linear galaxy bias parameters are a weighted version of bhO+RgO, where bhO is the halo bias, but the contribution from RgO is routinely ignored in the literature. We investigate the impact of this by measuring the RgO in separate universe simulations of the IllustrisTNG model for three types of perturbations: total matter perturbations, O=δm; baryon-CDM compensated isocurvature perturbations, O=σ; and potential perturbations with local primordial non-Gaussianity, O∝fNLϕ. Our main takeaway message is that the RgO are not negligible in general and their size should be estimated on a case-by-case basis. For stellar-mass selected galaxies, the responses Rgϕ and Rgσ are sizeable and cannot be neglected in HM calculations of the bias parameters bgϕ and bgσ; this is relevant to constrain inflation using galaxies. On the other hand, we do not detect a strong impact of the HOD response Rg1 on the linear galaxy bias bg1. These results can be explained by the impact that the perturbations have on stellar-to-total-mass relations. We also look into the impact on the bias of the gas distribution and find similar conclusions. We show that a single extra parameter describing the overall amplitude of RgO recovers the measured bgO well, which indicates that RgO can be easily added to HM/HOD studies as a new ingredient.