Star Formation Histories of the LEGUS dwarf galaxies. II. Spatially
  resolved star formation history of the Magellanic irregular NGC 4449
  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 ($D=3.82\pm 0.27$ Mpc) we
reach stars 3 magnitudes fainter than the tip of the red giant branch in the
F814W filter. The recovered star formation history 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. $\sim 3$
Gyr. The most recent peak of star formation is around 10 Myr ago. The average
surface density star formation rate over the whole galaxy lifetime is $0.01$
M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$. From our study it emerges that NGC 4449 has
experienced a fairly continuous star formation regime in the last 1 Gyr with
peaks and dips whose star formation 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 star formation history does
indeed provide some hints on the connection between these two phenomena in this
peculiar dwarf galaxy.
