The star formation history of the starburst region NGC 2363 and its surroundings

We present Hubble Space Telescope optical images and UV spectra, as well as ground-based near-infrared images of the high surface-brightness giant H II region NGC 2363 (NGC 2366-I) and its surroundings. The massive star content of the southern end of the dwarf irregular galaxy NGC 2366 is investigated, with an emphasis on Wolf-Rayet and red supergiant stars, and we attempt the reconstruction of the time sequence of the most recent episode of massive star formation at the southwestern tip of the galaxy. The ages of the clusters are, respectively, 10 Myr for NGC 2366-II, 2.5-5 Myr for NGC 2363-B, and less than 10 Myr for NGC 2363-A. In particular, we show that the most massive supercluster A of NGC 2363 is still embedded in dust; from the photoevaporative erosion or "cleaning" timescale of the associated cloud, we infer its age to be similar to 10(6) yr or less. We conclude that the star-forming complex NGC 2366-I and 2366-II is a good example of a multiple stage starburst with a characteristic age decreasing from 10 Myr to less than 1 Myr over a linear scale of 400 pc. The age sequence of the stars and the gas kinematics suggest that these powerful star formation episodes are being triggered by a small passing-by satellite.