Star Formation in Ultrafaint Dwarfs: Continuous or Single-age Bursts?

TL;DR

This study models the chemical evolution of six ultrafaint dwarf galaxies, showing that continuous or extended star formation with self-enrichment best explains their observed chemical properties, rather than single-age bursts.

Contribution

It introduces a continuous star formation model with short interruptions, providing a better fit to observed chemical abundances than single-burst scenarios.

Findings

Single-age bursts cannot explain observed [$ ext{α}$/Fe] vs [Fe/H] distributions.

Continuous star formation with self-enrichment reproduces chemical properties.

Ultrafaint dwarfs likely formed at low mass, not as remnants of larger systems.

Abstract

We model the chemical evolution of six UFDs: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV and Ursa Major I, based on their recently determined star formation histories. We show that two single-age bursts cannot explain the observed [$\alpha$/Fe] vs [Fe/H] distribution in these galaxies and that some self-enrichment is required within the first burst. An alternative scenario is modelled, in which star formation is continuous except for short interruptions when one or more supernovae temporarily blow the dense gas out from the centre of the system. This model allows for self-enrichment and can reproduce the chemical abundances of the UFDs in which the second burst is only a trace population. We conclude that the most likely star formation history is one or two extended periods of star formation, with the first burst lasting for at least 100 Myr. As found in earlier work, the observed properties of UFDs can be explained by formation at a low mass ($M_{\rm{vir}}\sim10^7$ M$_\odot$), rather than being stripped remnants of much larger systems.