Non-Separable Halo Bias from High-Redshift Galaxy Clustering

TL;DR

This paper tests the assumption of halo bias separability in galaxy clustering models and finds significant deviations at high redshifts on quasi-linear scales, impacting the interpretation of galaxy-halo connections.

Contribution

It introduces a method to quantify halo bias separability deviations and demonstrates their significance at high redshifts using simulations and observational data.

Findings

Significant departures from bias separability at high redshifts on quasi-linear scales.

Deviations lead to a suppression of halo cross-correlations by up to a factor of 2.

High-redshift galaxy cross-correlations can probe non-separability to improve galaxy-halo models.

Abstract

The halo model provides a powerful framework for interpreting galaxy clustering by linking the spatial distribution of dark matter haloes to the underlying matter distribution. A key assumption within the halo bias approximation of the halo model is that, on sufficiently large scales, the halo bias between two halo populations is a separable function of the mass of each population. In this work, we test the validity of this approximation on quasi-linear scales using both simulations and observational data across a broad range of halo masses and redshifts. In particular, we define a separability function based on halo or galaxy cross-correlations to quantify deviations from halo bias separability, and measure it from N-body simulations. We find significant departures from separability on quasi-linear scales (\(\sim 1\text{--}5\,\mathrm{Mpc}\)) at high redshifts (\(z \geq 3\)), leading to a suppression in the scale-dependent halo bias and hence in halo cross-correlations by up to a factor of 2 -- or even higher. In contrast, deviations at low redshifts remain modest. Additionally, using high-redshift (\(z \sim 3.6\)) galaxy samples, we detect deviations from bias separability that closely align with simulation predictions. The breakdown of the separable bias approximation on quasi-linear scales at high redshifts underscore the importance to account for non-separability in models of the galaxy-halo connection in this regime. Furthermore, these results highlight the potential of high-redshift galaxy cross-correlations as a probe for improving the galaxy-halo connection from upcoming large-scale surveys.