# Cosmic Reionization On Computers: Reionization Histories of Present-day Galaxies

## TL;DR

This study investigates the reionization histories of present-day galaxies by tracking their building blocks through cosmological simulations, revealing mass-dependent reionization timelines and internal spread.

## Contribution

It introduces a method to trace halo building blocks' reionization histories and analyzes their mass dependence and internal variation.

## Key findings

- Higher mass halos reionize earlier.
- Reionization times vary within halos based on mass.
- Reionization redshift correlates with galaxy mass.

## Abstract

We examine the reionization history of present-day galaxies by explicitly tracing the building blocks of halos from the Cosmic Reionization On Computers project. We track dark matter particles that belong to $z=0$ halos to trace the neutral fractions at corresponding positions during rapid global reionization. The resulting particle reionization histories allow us to explore different definitions of a halo's reionization redshift and to account for the neutral content of the interstellar medium. Consistent with previous work, we find a systematic trend of reionization redshift with mass - present day halos with higher masses have earlier reionization times. Finally, we quantify the spread of reionization times within each halo, which also has a mass dependence.