Scale-dependent halo bias and the squeezed limit bispectrum in the presence of radiation

TL;DR

This paper examines how large-scale radiation perturbations influence small-scale structure formation, revealing a scale-dependent effect on matter growth and halo bias at large scales through simulations.

Contribution

It introduces a novel analysis of radiation's impact on scale-dependent halo bias and bispectrum using Separate Universe N-body simulations.

Findings

Radiation induces non-trivial scale dependence in matter growth and halo bias.

Bias decreases by up to 0.8% at large scales due to radiation effects.

Results quantify the scale-dependent bias change across different redshifts.

Abstract

We investigate the gravitational effect of large-scale radiation perturbations on small-scale structure formation. In addition to making the growth of matter perturbations scale dependent, the free-streaming of radiation also affects the coupling between structure formation at small and large scales. We study this using Separate Universe N-body simulations to compute the (isotropized) squeezed-limit matter bispectrum and the linear halo bias. Our results show that the scale dependence in the growth of long-wavelength matter perturbations, caused by radiation, translates into these quantities acquiring a non-trivial scale-dependence at $k\lesssim 0.05$ Mpc$^{-1}$. In a universe with radiation composed of cosmic microwave background photons and three species of massless neutrinos, the bias of halos with $b = 2$ at high $k$ will decrease by $0.29\%,\ 0.45\%$ and $0.8\%$ between $k = 0.05$ Mpc$^{-1}$ and $k = 0.0005$ Mpc$^{-1}$ at redshifts $z=0,\ 1$, and $3$ respectively. For objects with $b\gg1$, these differences approach $0.43\%,\ 0.68\%$ and $1.2\%$ respectively.