The triggering of starbursts in low-mass galaxies
  Strong bursts of star formation in galaxies may be triggered either by
internal or external mechanisms. We study the distribution and kinematics of
the HI gas in the outer regions of 18 nearby starburst dwarf galaxies, that
have accurate star-formation histories from HST observations of resolved
stellar populations. We find that starburst dwarfs show a variety of HI
morphologies, ranging from heavily disturbed HI distributions with major
asymmetries, long filaments, and/or HI-stellar offsets, to lopsided HI
distributions with minor asymmetries. We quantify the outer HI asymmetry for
both our sample and a control sample of typical dwarf irregulars. Starburst
dwarfs have more asymmetric outer HI morphologies than typical irregulars,
suggesting that some external mechanism triggered the starburst. Moreover,
galaxies hosting an old burst (>100 Myr) have more symmetric HI morphologies
than galaxies hosting a young one (<100 Myr), indicating that the former ones
probably had enough time to regularize their outer HI distribution since the
onset of the burst. We also investigate the nearby environment of these
starburst dwarfs and find that most of them ($\sim$80$\%$) have at least one
potential perturber at a projected distance <200 kpc. Our results suggest that
the starburst is triggered either by past interactions/mergers between gas-rich
dwarfs or by direct gas infall from the IGM.
