On the extended structure of the Phoenix dwarf galaxy
  We present the star formation history (SFH) and its variations with
galactocentric distance for the Local Group dwarf galaxy of Phoenix.
Color-magnitude diagram was obtained from WFPC2@HST reaching the oldest main
sequence turnoffs. The IAC-star and IAC-pop codes and the MinnIAC suite have
been used to obtain the star formation rate as a function of time, metallicity,
and radius. We find that Phoenix has had ongoing but gradually decreasing star
formation over nearly a Hubble time. The highest level of star formation
occurred from the formation of the galaxy till 10.5 Gyr ago, when 50% of the
total star formation had already taken place. From that moment, star formation
continues at a significant level until 6 Gyr ago, and at a very low level till
the present time. The chemical enrichment law shows a trend of slowly
increasing metallicity as a function of time till 8--6 Gyr ago, when Z starts
to increase steeply to the current value.
  Young stars are found in the inner region of the galaxy only, but
intermediate-age and old stars can be found at all galactocentric distances.
This study shows that star formation started at all galactocentric distances in
Phoenix at an early epoch. Our results are compatible with a scenario in which
the star formation region envelope slowly shrinks as time goes on, possibly as
a natural result of pressure support reduction as gas supply diminishes. As a
consequence, star formation stopped first in outer regions and the scale-length
of the stellar mass density distribution decreased with time. No traces of a
true, old halo are apparent in Phoenix either in its stellar age distribution
or in the stellar mass density distribution, at least out to 0.5 kpc (about 2.5
scale-lengths) from the center.
