The VMC survey – XIV. First results on the look-back time star formation rate tomography of the Small Magellanic Cloud★

Abstract

We analyse deep images from the VISTA survey of the Magellanic Clouds in the\nYJKs filters, covering 14 sqrdeg (10 tiles), split into 120 subregions, and\ncomprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply\na colour--magnitude diagram reconstruction method that returns their\nbest-fitting star formation rate SFR(t), age-metallicity relation (AMR),\ndistance and mean reddening, together with 68% confidence intervals. The\ndistance data can be approximated by a plane tilted in the East-West direction\nwith a mean inclination of 39 deg, although deviations of up to 3 kpc suggest a\ndistorted and warped disk. After assigning to every observed star a probability\nof belonging to a given age-metallicity interval, we build high-resolution\npopulation maps. These dramatically reveal the flocculent nature of the young\nstar-forming regions and the nearly smooth features traced by older stellar\ngenerations. They document the formation of the SMC Wing at ages <0.2 Gyr and\nthe peak of star formation in the SMC Bar at 40 Myr. We clearly detect periods\nof enhanced star formation at 1.5 Gyr and 5 Gyr. The former is possibly related\nto a new feature found in the AMR, which suggests ingestion of metal-poor gas\nat ages slightly larger than 1 Gyr. The latter constitutes a major period of\nstellar mass formation. We confirm that the SFR(t) was moderately low at even\nolder ages.\n