Star Formation Histories of the LEGUS Dwarf Galaxies. II. Spatially Resolved Star Formation History of the Magellanic Irregular NGC 4449*

摘要

We present a detailed study of the Magellanic irregular galaxy NGC?4449 based on both archival and new photometric data from the Legacy Extragalactic UV Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3. Thanks to its proximity (D = 3.82 ± 0.27 Mpc), we reach stars 3 mag fainter than the tip of the red giant branch in the F814W filter. The recovered star formation history (SFH) spans the whole Hubble time, but due to the age–metallicity degeneracy of the red giant branch stars, it is robust only over the lookback time reached by our photometry, i.e., ~3?Gyr. The most recent peak of star formation (SF) is around 10 Myr ago. The average surface density SF rate over the whole galaxy lifetime is 0.01?M ⊙ yr?1 kpc?2. From our study, it emerges that NGC?4449 has experienced a fairly continuous SF regime in the last 1 Gyr, with peaks and dips whose SF rates differ only by a factor of a few. The very complex and disturbed morphology of NGC?4449 makes it an interesting galaxy for studies of the relationship between interactions and starbursts, and our detailed and spatially resolved analysis of its SFH does indeed provide some hints on the connection between these two phenomena in this peculiar dwarf galaxy.