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