Variability and star formation in Leo T, the lowest luminosity star-forming galaxy known today

TL;DR

This study combines variable star analysis and star formation history to reveal Leo T's complex evolution, including multiple star formation episodes and its transitional galaxy status, using Hubble data.

Contribution

First combined analysis of variable stars and star formation history in Leo T, revealing multiple star formation episodes and its transitional nature.

Findings

Detected 14 variable stars including RR Lyrae and Anomalous Cepheids.

Estimated distance to Leo T as 409 kpc.

Confirmed complex star formation history with two main periods.

Abstract

We present results from the first combined study of variable stars and star formation history (SFH) of the Milky Way (MW) "ultra-faint" dwarf (UFD) galaxy Leo T, based on F606W and F814W multi-epoch archive observations obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We have detected 14 variable stars in the galaxy. They include one fundamental-mode RR Lyrae star and 10 Anomalous Cepheids with periods shorter than 1 day, thus suggesting the occurrence of multiple star formation episodes in this UFD, of which one about 10 Gyr ago produced the RR Lyrae star. A new estimate of the distance to Leo T of 409 $^{+29}_{-27}$ kpc (distance modulus of 23.06 $\pm$ 0.15 mag) was derived from the galaxy's RR Lyrae star. Our V, V-I color-magnitude diagram of Leo T reaches V~29 mag and shows features typical of a galaxy in transition between dwarf irregular and dwarf spheroidal types. A quantitative analysis of the star formation history, based on the comparison of the observed V,V-I CMD with the expected distribution of stars for different evolutionary scenarios, confirms that Leo T has a complex star formation history dominated by two enhanced periods about 1.5 and 9 Gyr ago, respectively. The distribution of stars and gas shows that the galaxy has a fairly asymmetric structure.