Ongoing star formation at the outskirts of Sextans A: Spectroscopic detection of early-O type stars.

Abstract

With both nebular- and stellar-derived abundances of $\lesssim$ 1/10 Zsun and low foreground extinction, Sextans A is a prime candidate to replace the Small Magellanic Cloud as reservoir of metal-poor massive stars and reference to study the metal-poor Universe. We report the discovery of two early-O type stars in Sextans A, the earliest O-stars with metallicity < 1/7 Zsun known to date, and two additional O9 stars. Colour-excess estimates towards individual targets, enabled by spectral typing, manifest that internal reddening is neither uniform nor negligible. The four targets define a new region of star formation far from the optically-brightest centre of the galaxy and from its conspicuous HII shells, but not devoid of neutral hydrogen. In fact, we detect a spatial correlation between OB-stars and HI in Sextans A and other dIrr's that leads us to propose that the neutral phase may be fundamental to star formation in low-density environments. According to the existing evidence at least two of the targets formed in isolation, thus suggestive of an stochastic sampling of the initial mass function that would enable low-mass galaxies like Sextans A to form very massive stars. The discovery of these four stars provide spatially-resolved, spectroscopic confirmation of recent findings suggesting that dwarf galaxies can sustain star formation despite the low density of the gas phase.