# The HI Bias during the Epoch of Reionization

## TL;DR

This paper investigates how neutral hydrogen traces matter during the epoch of reionization using semi-numerical models, revealing scale-dependent bias evolution and a transition scale in the 21 cm line signal.

## Contribution

It introduces a combined modeling approach to study HI bias evolution and scale dependence during reionization, highlighting a transition scale in the 21 cm bias.

## Key findings

- Neutral hydrogen bias is negative on large scales during reionization.
- The magnitude of the bias increases early and decreases late in reionization.
- A transition scale exists where HI bias shifts from negative to positive.

## Abstract

The neutral hydrogen (HI) and its 21 cm line are promising probes to the reionization process of the intergalactic medium (IGM). To use this probe effectively, it is imperative to have a good understanding on how the neutral hydrogen traces the underlying matter distribution. Here we study this problem using semi-numerical modeling by combining the HI in the IGM and the HI from halos during the epoch of reionization (EoR), and investigate the evolution and the scale-dependence of the neutral fraction bias as well as the 21 cm line bias. We find that the neutral fraction bias on large scales is negative during reionization, and its absolute value on large scales increases during the early stage of reionization and then decreases during the late stage. During the late stage of reionization, there is a transition scale at which the HI bias transits from negative on large scales to positive on small scales, and this scale increases as the reionization proceeds to the end.