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

Abstract

Abstract 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 R 𝒪 g that describe the response of the HODs to long-wavelength perturbations 𝒪. The linear galaxy bias parameters b 𝒪 g are a weighted version of b 𝒪 h + R 𝒪 g , where b 𝒪 h is the halo bias, but the contribution from R 𝒪 g is routinely ignored in the literature. We investigate the impact of this by measuring the R 𝒪 g in separate universe simulations of the IllustrisTNG model for three types of perturbations: total matter perturbations, 𝒪 = δ h ; baryon-CDM compensated isocurvature perturbations, 𝒪 = σ; and potential perturbations with local primordial non-Gaussianity, 𝒪 ∝ f NL ϕ. Our main takeaway message is that the R 𝒪 g are not negligible in general and their size should be estimated on a case-by-case basis. For stellar-mass selected galaxies, the responses R 𝒪 g and R σ g are sizeable and cannot be neglected in HM calculations of the bias parameters b ϕ g and b σ g ; this is relevant to constrain inflation using galaxies. On the other hand, we do not detect a strong impact of the HOD response R 1 g on the linear galaxy bias b 1 g . These results can be explained by the impact that the perturbations 𝒪 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 R 𝒪 g recovers the measured b 𝒪 g well, which indicates that R 𝒪 g can be easily added to HM/HOD studies as a new ingredient.

Topics & Concepts