The MUSE-Faint survey

IV. Dissecting Leo T, a gas-rich relic with recent star formation

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: daniel.vaz@astro.up.pt
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal
³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁶ University of Surrey, Physics Department, Guildford GU2 7XH, UK
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁸ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany

Received: 19 March 2023
Accepted: 4 August 2023

Abstract

Context. Leo T (M_V = −8.0) is a peculiar dwarf galaxy that stands out for being both the faintest and the least massive galaxy known to contain neutral gas and to display signs of recent star formation. It is also extremely dark-matter dominated. As a result, Leo T presents an invaluable opportunity to study the processes of gas and star formation at the limit where galaxies are found to have rejuvenating episodes of star formation.

Aims. Our approach to studying Leo T involves analysing photometry and stellar spectra to identify member stars and gather information about their properties, such as line-of-sight velocities, stellar metallicities, and ages. By examining these characteristics, we aim to better understand the overall dynamics and stellar content of the galaxy and to compare the properties of its young and old stars.

Methods. Our study of Leo T relies on data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, which we use to identify 58 member stars of the galaxy. In addition, we supplement this information with spectroscopic data from the literature to bring the total number of member stars analysed to 75. To further our analysis, we complement these data with Hubble Space Telescope (HST) photometry. With these combined datasets, we delve deeper into the galaxy’s stellar content and uncover new insights into its properties.

Results. Our analysis reveals two distinct populations of stars in Leo T. The first population, with an age of ≲500 Myr, includes three emission-line Be stars comprising 15% of the total number of young stars. The second population of stars is much older, with ages ranging from > 5 Gyr to as high as 10 Gyr. We combine MUSE data with literature data to obtain an overall velocity dispersion of σ_v = 7.07_−1.12^+1.29 km s⁻¹ for Leo T. When we divide the sample of stars into young and old populations, we find that they have distinct kinematics. Specifically, the young population has a velocity dispersion of 2.31_−1.65^+2.68 km s⁻¹, contrasting with that of the old population, of 8.14_−1.38^+1.66 km s⁻¹. The fact that the kinematics of the cold neutral gas is in good agreement with the kinematics of the young population suggests that the recent star formation in Leo T is linked with the cold neutral gas. We assess the existence of extended emission-line regions and find none to a surface brightness limit of < 1 × 10⁻²⁰ erg s⁻¹ cm⁻² arcsec⁻² which corresponds to an upper limit on star formation of ∼10⁻¹¹ M_⊙ yr⁻¹ pc⁻², implying that the star formation in Leo T has ended.