A detailed star formation history for the extremely diffuse Andromeda XIX dwarf galaxy

TL;DR

This study uses deep Hubble imaging to determine that Andromeda XIX, an ultra-diffuse dwarf galaxy, stopped forming stars around 9.7 billion years ago, highlighting its unique properties and potential formation mechanisms.

Contribution

It provides the first detailed star formation history of Andromeda XIX, revealing its early quenching and challenging existing formation models for such diffuse galaxies.

Findings

Star formation ceased approximately 9.7 Gyr ago.

Galaxy exhibits extremely large half-light radius and low-density dark matter halo.

Early quenching rules out feedback from bursty star formation as the cause.

Abstract

We present deep imaging of the ultra-diffuse Andromeda XIX dwarf galaxy from the Advance Camera for Surveys on the Hubble Space Telescope which resolves its stellar populations to below the oldest main sequence turn-off. We derive a full star formation history for the galaxy using MATCH, and find no evidence of star formation in the past 8 Gyr. We calculate a quenching time of $\tau_{90}=9.7\pm0.2$~Gyr, suggesting Andromeda~XIX ceased forming stars very early on. This early quenching, combined with its extremely large half-light radius, low density dark matter halo and lower than expected metallicity make it a unique galaxy within the Local Group and raises questions about how it formed. The early quenching time allows us to rule out feedback from bursty star formation as a means to explain its diffuse stellar population and low density dark matter halo. We find that the extended stellar population, low density halo and star formation could be explained by either tidal interactions (such as tidal shocking) or by late dry mergers, with the latter also explaining its low metallicity. Proper motions and detailed abundances would allow us to distinguish between these two scenarios.