# Imprints of Reionization in Galaxy Clustering

## TL;DR

This paper investigates how the epoch of reionization leaves detectable imprints on galaxy clustering, focusing on effects like baryon-CDM relative velocities and ionizing radiation modulation, which are crucial for precise cosmological measurements.

## Contribution

The study derives the reionization imprints within the large-scale bias expansion and constrains these effects using BOSS DR12 galaxy data, highlighting their importance for unbiased cosmology.

## Key findings

- Reionization induces a galaxy bias involving matter velocity squared.
- Ionizing radiation modulates galaxy density through higher-derivative bias terms.
- Including these effects is essential for accurate cosmological parameter estimation.

## Abstract

Reionization, the only phase transition in the Universe since recombination, is a key event in the cosmic history of baryonic matter. We derive, in the context of the large-scale bias expansion, the imprints of the epoch of reionization in the large-scale distribution of galaxies, and identify two contributions of particular importance. First, the Compton scattering of CMB photons off the free electrons lead to a drag force on the baryon fluid. This drag induces a relative velocity between baryons and CDM which is of the same order of magnitude as the primordially-induced relative velocity, and enters in the evolution of the relative velocity as calculated by Boltzmann codes. This leads to a unique contribution to galaxy bias involving the matter velocity squared. The second important effect is a modulation of the galaxy density by the ionizing radiation field through radiative-transfer effects, which is captured in the bias expansion by so-called higher-derivative terms. We constrain both of these imprints using the power spectrum of the BOSS DR12 galaxy sample. While they do not lead to a shift in the baryon acoustic oscillation scale, including these terms is important for unbiased cosmology constraints from the shape of the galaxy power spectrum.