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 $R_mathcal{O}^g$ that describe the response of the HODs to long-wavelength 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, $mathcal{O}=delta_m$; baryon-CDM compensated isocurvature perturbations, $mathcal{O}=sigma$; and potential perturbations with local primordial non-Gaussianity, $mathcal{O}propto f_{rm NL}phi$. Our main takeaway message is that the $R_mathcal{O}^g$ are not negligible in general and their size should be estimated on a case-by-case basis. For stellar-mass 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 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 $R_mathcal{O}^g$ 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.
تحميل البحث