Responses of Halo Occupation Distributions: a new ingredient in the halo model & the impact on galaxy bias
Abstract
Halo occupation distribution (HOD) models describe the number of galaxies that reside in different haloes, and are widely used in galaxyhalo connection studies using the halo model (HM). Here, we introduce and study HOD response functions $R_\mathcal{O}^g$ that describe the response of the HODs to longwavelength perturbations $\mathcal{O}$. The linear galaxy bias parameters $b_\mathcal{O}^g$ are a weighted version of $b_\mathcal{O}^h + R_\mathcal{O}^g$, where $b_\mathcal{O}^h$ is the halo bias, but the contribution from $R_\mathcal{O}^g$ is routinely ignored in the literature. We investigate the impact of this by measuring the $R_\mathcal{O}^g$ in separate universe simulations of the IllustrisTNG model for three types of perturbations: total matter perturbations, $\mathcal{O}=\delta_m$; baryonCDM compensated isocurvature perturbations, $\mathcal{O}=\sigma$; and potential perturbations with local primordial nonGaussianity, $\mathcal{O}\propto f_{\rm NL}\phi$. Our main takeaway message is that the $R_\mathcal{O}^g$ are not negligible in general and their size should be estimated on a casebycase basis. For stellarmass selected galaxies, the responses $R_\phi^g$ and $R_\sigma^g$ are sizeable and cannot be neglected in HM calculations of the bias parameters $b_\phi^g$ and $b_\sigma^g$; 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 have on stellartototalmass 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 $R_\mathcal{O}^g$ recovers the measured $b_\mathcal{O}^g$ well, which indicates that $R_\mathcal{O}^g$ can be easily added to HM/HOD studies as a new ingredient.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 May 2021
 DOI:
 10.1088/14757516/2021/05/069
 arXiv:
 arXiv:2012.04637
 Bibcode:
 2021JCAP...05..069V
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 22 pages, 12 figures, 1 table. Comments are welcome! Accepted by JCAP