Recent star formation history of the dwarf irregular galaxy Leo A

Abstract

Context.Leo A is a gas-rich dwarf irregular galaxy of low stellar mass and metallicity. Its star formation history extends up to ∼10 Gyr. A few prominent H II regions indicate that star formation processes are active in the galaxy to this day. In the present epoch, Leo A is a highly isolated galaxy, which allows for it to be studied in a self-propagating star formation regime. Aims. Our aim is to study the recent star formation history of the Leo A galaxy over the last ∼300 Myr. Methods. We analysed populations of main sequence (MS) and blue helium-burning (BHeB – ‘blue loop’) stars using multi-colour photometry data obtained with the Subaru/Suprime-Cam (B, V, R, I, Hα) and the Hubble Space Telescope (HST) Advanced Camera for Surveys (F475W and F814W) cameras. We made use of colour-magnitude diagrams and stellar isochrones to determine individual ages of the BHeB stars, which enabled us to study the spatial distribution of these stars in different age bins. In addition, we used Hα emission and H I column density maps to study the relationship between young stellar populations and the surrounding interstellar medium. Results. We found that significant differences are visible in the morphology of stellar distributions represented by the BHeB and MS stars of different ages. We also studied a well-known H I hole in the Leo A galaxy and found that there is a noticeable difference in the surface number density of stars in the western and eastern parts of the hole. The bright young (< 20 Myr) MS stars residing in the western part of the hole indicate the recent star-forming activity in this region after the quiescent period of ∼300 Myr. Furthermore, there is a shock front (prominent in Hα) that closely resembles the shape of the western edge of the H I hole. This shock front could have been formed by the combined stellar feedback from the young MS stars or a Type II supernova located within the H I hole. Additionally, an analysis of the Suprime-Cam photometry in Hα and R passbands enabled us to identify 17 stars with a prominent Hα emission (Hα − R ≤ −0.06) which indicate the presence of early-type (Be, B[e], or A-shell) emission-line stars in the Leo A galaxy.