# A new scale in the bias expansion

## TL;DR

This paper introduces a new scale in galaxy bias expansion caused by ionizing radiation's mean free path, affecting large-scale galaxy clustering and potentially impacting cosmological measurements.

## Contribution

It proposes a non-perturbative model to incorporate radiative-transfer effects into galaxy bias, enhancing the robustness of large-scale structure analyses.

## Key findings

- Radiative transfer introduces a significant new scale in galaxy clustering.
- Higher-derivative bias terms may be non-negligible due to ionizing radiation effects.
- A non-perturbative model can preserve the predictive power of galaxy clustering measurements.

## Abstract

The fact that the spatial nonlocality of galaxy formation is controlled by some short length scale like the Lagrangian radius is the cornerstone of the bias expansion for large-scale-structure tracers. However, the first sources of ionizing radiation between $z\approx 15$ and $z\approx 6$ are expected to have significant effects on the formation of galaxies we observe at lower redshift, at least on low-mass galaxies. These radiative-transfer effects introduce a new scale in the clustering of galaxies, i.e. the finite distance which ionizing radiation travels until it reaches a given galaxy. This mean free path can be very large, of order $100\,h^{-1}\,{\rm Mpc}$. Consequently, higher-derivative terms in the bias expansion could turn out to be non-negligible even on these scales: treating them perturbatively would lead to a massive loss in predictivity and, for example, could spoil the determination of the BAO feature or constraints on the neutrino mass. Here, we investigate under what assumptions an explicit non-perturbative model of radiative-transfer effects can maintain the robustness of large-scale galaxy clustering as a cosmological probe.