HST/ACS Photometry of Old Stars in NGC 1569: The Star Formation History
  of a Nearby Starburst
  (abridged) We used HST/ACS to obtain deep V- and I-band images of NGC 1569,
one of the closest and strongest starburst galaxies in the Universe. These data
allowed us to study the underlying old stellar population, aimed at
understanding NGC 1569's evolution over a full Hubble time. We focus on the
less-crowded outer region of the galaxy, for which the color-magnitude diagram
(CMD) shows predominantly a red giant branch (RGB) that reaches down to the red
clump/horizontal branch feature (RC/HB). A simple stellar population analysis
gives clear evidence for a more complicated star formation history (SFH) in the
outer region. We derive the full SFH using a newly developed code, SFHMATRIX,
which fits the CMD Hess diagram by solving a non-negative least squares
problem. Our analysis shows that the relative brightnesses of the RGB tip and
RC/HB, along with the curvature and color of the RGB, provide enough
information to ameliorate the age-metallicity-extinction degeneracy. The
distance/reddening combination that best fits the data is E(B-V) = 0.58 +/-
0.03 and D = 3.06 +/- 0.18 Mpc. Star formation began ~ 13 Gyr ago, and this
accounts for the majority of the mass in the outer region. However, the initial
burst was followed by a relatively low, but constant, rate of star formation
until ~ 0.5-0.7 Gyr ago when there may have been a short, low intensity burst
of star formation.
