Ultra faint dwarfs: probing early cosmic star formation

TL;DR

This paper models ultra faint dwarf galaxies within a cosmological framework, explaining their properties and formation history, and successfully reproduces observed metallicity and luminosity relations.

Contribution

It extends a merger tree model to include star-forming minihaloes and radiative feedback, providing new insights into the formation and characteristics of ultra faint dwarf galaxies.

Findings

UFs are the oldest, most dark matter-dominated dSphs formed before reionization.

Their MDF is broader and shifted towards lower metallicity compared to classical dSphs.

UFs are very inefficient at star formation, converting less than 3% of baryons into stars by z=0.

Abstract

We investigate the nature of the newly discovered Ultra Faint dwarf spheroidal galaxies (UF dSphs) in a general cosmological context simultaneously accounting for various ``classical`` dSphs and Milky Way properties including their Metallicity Distribution Function (MDF). To this aim we extend the merger tree approach previously developed to include the presence of star-forming minihaloes, and an heuristic prescription for radiative feedback. The model successfully reproduces both the observed [Fe/H]-Luminosity relation and the mean MDF of UFs. In this picture UFs are the oldest, most dark matter-dominated (M/L > 100) dSphs with a total mass M= 10^{7-8}Msun; they are leftovers of H_2-cooling minihaloes formed at z > 8.5, i.e. before reionization. Their MDF is broader (because of a more prolonged SF) and shifted towards lower [Fe/H] (as a result of a lower gas metallicity at the time of formation) than that of classical dSphs. These systems are very ineffectively star-forming, turning into stars by z=0 only <3% of the potentially available baryons. We provide a useful fit for the star formation efficiency of dSphs.