EDGE: The origin of scatter in ultra-faint dwarf stellar masses and surface brightnesses

TL;DR

This study uses hydrodynamical simulations to show that ultra-faint dwarf galaxies exhibit significant scatter in stellar mass at fixed halo mass due to diverse formation histories, especially around reionization.

Contribution

It introduces a new formation scenario where late stellar mass growth occurs mainly through dry mergers, explaining the properties of diffuse ultra-faint dwarfs.

Findings

Extended scatter (≥1 dex) in stellar mass at fixed halo mass.

Early formation leads to higher stellar mass before reionization.

Late stellar growth via dry mergers results in diffuse, metal-poor ultra-faint dwarfs.

Abstract

We demonstrate how the least luminous galaxies in the Universe, ultra-faint dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic reionization. We select a low-mass ($\sim \text{1.5} \times 10^{9} \, \text{M}_{\odot}$) dark matter halo from a cosmological volume, and perform zoom hydrodynamical simulations with multiple alternative histories using "genetically modified" initial conditions. Earlier forming ultra-faints have higher stellar mass today, due to a longer period of star formation before their quenching by reionization. Our histories all converge to the same final dynamical mass, demonstrating the existence of extended scatter ($\geq$ 1 dex) in stellar masses at fixed halo mass due to the diversity of possible histories. One of our variants builds less than 2 % of its final dynamical mass before reionization, rapidly quenching in-situ star formation. The bulk of its final stellar mass is later grown by dry mergers, depositing stars in the galaxy's outskirts and hence expanding its effective radius. This mechanism constitutes a new formation scenario for highly diffuse ($\text{r}_{1 /2} \sim 820 \, \text{pc}$, $\sim 32 \, \text{mag arcsec}^2$), metal-poor ($\big[ \mathrm{Fe}\, / \mathrm{H} \big]= -2.9$), ultra-faint ($\mathcal{M}_V= -5.7$) dwarf galaxies within the reach of next-generation low surface brightness surveys.