Previous studies of Ultra-Faint Dwarfs (UFDs) show that their dynamical mass-to–light ratios are the highest values measured in any type of galaxy, implying relatively pure dark-matter halos with minimal baryonic content. Furthermore, UFDs have the lowest metallicities, oldest ages, smallest sizes, and simplest assembly histories of all galaxies. Understanding the nature of these galaxies would improve immensely the knowledge of the galaxy formation process and help to unravel the nature of dark matter.
The Leo T UFD is one of the lowest mass galaxies known to contain neutral gas and to be also extremely dark-matter dominated. Previous studies have shown signs of recent star formation in the galaxy, making it one of the very faintest galaxies to show this. It is, therefore, an interesting laboratory for studies of gas and star formation at the limit of where galaxies are found to have rejuvenating episodes of star formation.
In this contribution I will discuss a novel study of Leo T that we have done using data from the MUSE integral field spectrograph. The high sensitivity of MUSE allowed us to obtain velocity measurements for stars as faint as magnitude ~24, which allowed us to increase the number of Leo T stars observed spectroscopically from 19 to 75.
Combining the MUSE data with photometric data from HST, we have studied the age and metallicity of these stars and identified two populations, all consistent with similar metallicity. Within the young population sample we discovered three emission line Be stars - a first for ultra-faint dwarfs.
While looking for differences in the dynamics of young and old stars, we find that they have different kinematics, with the young population having a velocity dispersion consistent with the kinematics of the cold component of the neutral gas.
In this contribution I will discuss these results and their implications for the origin and evolution of Leo T.
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