# Clues to the nature of dark matter from first galaxies

## TL;DR

This study uses high-resolution galaxy formation simulations to explore how a 3 keV warm dark matter model affects early universe galaxy properties, reionization, and observable signatures compared to cold dark matter.

## Contribution

It provides the first detailed simulation-based analysis of warm dark matter effects on high-redshift galaxy formation and reionization, highlighting key differences from cold dark matter.

## Key findings

- WDM suppresses low-mass halo formation.
- WDM results in fewer stars at fixed halo mass.
- WDM leads to higher dark halo fractions below 10^9 M_sun.

## Abstract

We use thirty-eight high-resolution simulations of galaxy formation between redshift 10 and 5 to study the impact of a 3 keV warm dark matter (WDM) candidate on the high-redshift Universe. We focus our attention on the stellar mass function and the global star formation rate and consider the consequences for reionization, namely the neutral hydrogen fraction evolution and the electron scattering optical depth. We find that three different effects contribute to differentiate warm and cold dark matter (CDM) predictions: WDM suppresses the number of haloes with mass less than few $10^9$ M$_{\odot}$; at a fixed halo mass, WDM produces fewer stars than CDM; and finally at halo masses below $10^9$ M$_{\odot}$, WDM has a larger fraction of dark haloes than CDM post-reionization. These three effects combine to produce a lower stellar mass function in WDM for galaxies with stellar masses at and below $\sim 10^7$ M$_{\odot}$. For $z > 7$, the global star formation density is lower by a factor of two in the WDM scenario, and for a fixed escape fraction, the fraction of neutral hydrogen is higher by 0.3 at $z \sim 6$. This latter quantity can be partially reconciled with CDM and observations only by increasing the escape fraction from 23 per cent to 34 per cent. Overall, our study shows that galaxy formation simulations at high redshift are a key tool to differentiate between dark matter candidates given a model for baryonic physics.