Discovery of a population of pre-main-sequence stars in NGC 346 from deep Hubble Space Telescope ACS images

We report the discovery of a rich population of low-mass stars in the young, massive NGC 346 star-forming region in the Small Magellanic Cloud from deep V, I, and H alpha images taken with the Hubble Space Telescope Advanced Camera for Surveys. These stars have likely formed together with the NGC 346 cluster, similar or equal to 3-5 Myr ago. Their magnitudes and colors are those of pre-main-sequence stars in the mass range 0.6-3 M(circle dot), mostly concentrated in the main cluster, but with secondary subclusters spread over a region across similar to 45 pc. These subclusters appear to be spatially coincident with previously known knots of molecular gas identified in ground-based and Infrared Space Observatory observations. We show that NGC 346 is a complex region, being shaped by its massive stars, and the observations presented here represent a key step toward the understanding of how star formation occurred and has progressed in this low-metallicity environment.