Discovery and Characterization of Two Ultra Faint-Dwarfs Outside the Halo of the Milky Way: Leo M and Leo K

TL;DR

This paper reports the discovery and detailed analysis of two ultra-faint dwarf galaxies, Leo M and Leo K, outside the Milky Way's halo, providing insights into their properties, star formation histories, and potential interactions within the Local Group.

Contribution

It presents the first detailed characterization of Leo M and Leo K, including their structural parameters, distances, stellar masses, and star formation histories, highlighting their early quenching and non-satellite status.

Findings

Leo M and Leo K are ultra-faint dwarf galaxies outside the Milky Way halo.

Both galaxies were quenched over 10 billion years ago.

Their early quenching is consistent with reionization and stellar feedback effects.

Abstract

We report the discovery of two ultra-faint dwarf galaxies, Leo M and Leo K, that lie outside the halo of the Milky Way. Using Hubble Space Telescope imaging of the resolved stars, we create color-magnitude diagrams reaching the old main sequence turn-off of each system and (i) fit for structural parameters of the galaxies; (ii) measure their distances using the luminosity of the Horizontal Branch stars; (iii) estimate integrated magnitudes and stellar masses; and (iv) reconstruct the star formation histories. Based on their location in the Local Group, neither galaxy is currently a satellite of the Milky Way, although Leo K is located ~26 kpc from the low-mass galaxy Leo T and these two systems may have had a past interaction. Leo M and Leo K have stellar masses of 1.8 (+0.3/-0.2) x 10^4 Msun and 1.2+/-0.2 x 10^4 Msun, and were quenched 10.6 (+2.2/-1.1) Gyr and 12.8 (+0.1/-4.2) Gyr ago, respectively. Given that the galaxies are not satellites of the MW, it is unlikely that they were quenched by environmental processing. Instead, given their low stellar masses, their early quenching timescales are consistent with the scenario that a combination of reionization and stellar feedback shut-down star formation at early cosmic times.