Constraining the Warm Dark Matter Particle Mass through Ultra-Deep UV Luminosity Functions at z=2

TL;DR

This study constrains the mass of warm dark matter particles by analyzing ultra-deep UV luminosity functions at redshift 2, providing robust lower limits on particle mass based on galaxy abundance data.

Contribution

It offers new constraints on warm dark matter particle mass using deep UV galaxy observations, independent of star formation efficiency assumptions.

Findings

Lower limit of m_X > 1.8 keV for thermal relic WDM.

Lower limit of m_X > 1.5 keV from luminosity function fits.

Predictions for faint galaxy abundance at different m_X values.

Abstract

We compute the mass function of galactic dark matter halos for different values of the Warm Dark Matter (WDM) particle mass m_X and compare it with the abundance of ultra-faint galaxies derived from the deepest UV luminosity function available so far at redshift z~2. The magnitude limit M_UV=-13 reached by such observations allows us to probe the WDM mass functions down to scales close to or smaller than the half-mass mode mass scale ~10^9 M_sun. This allowed for an efficient discrimination among predictions for different m_X which turn out to be independent of the star formation efficiency adopted to associate the observed UV luminosities of galaxies to the corresponding dark matter masses. Adopting a conservative approach to take into account the existing theoretical uncertainties in the galaxy halo mass function, we derive a robust limit m_X>1.8 keV for the mass of thermal relic WDM particles when comparing with the measured abundance of the faintest galaxies, while m_X>1.5 keV is obtained when we compare with the Schechter fit to the observed luminosity function. The corresponding lower limit for sterile neutrinos depends on the modeling of the production mechanism; for instance m_sterile > 4 keV holds for the Shi-Fuller mechanism. We discuss the impact of observational uncertainties on the above bound on m_X. As a baseline for comparison with forthcoming observations from the HST Frontier Field, we provide predictions for the abundance of faint galaxies with M_UV=-13 for different values of m_X and of the star formation efficiency, valid up to z~4.