The Star Formation History in the M31 Bulge
  We present the study of stellar populations in the central 5.5' (~1.2 kpc) of
the M31 bulge by using the optical color magnitude diagram derived from HST ACS
WFC/HRC observations. In order to enhance image quality and then obtain deeper
photometry, we construct Nyquist-sampled images and use a deconvolution method
to detect sources and measure their photometry. We demonstrate that our method
performs better than DOLPHOT in the extremely crowded region. The resolved
stars in the M31 bulge have been divided into nine annuli and the color
magnitude diagram fitting is performed for each of them. We confirm that the
majority of stars (> 70%) in the M31 bulge are indeed very old (>5 Gyr) and
metal-rich ([Fe/H] > 0.3). At later times, the star formation rate decreased
and then experienced a significant rise around 1 Gyr ago, which pervaded the
entire M31 bulge. After that, stars formed at less than 500 Myr ago in the
central 130". Through simulation, we find that these intermediate-age stars
cannot be the artifacts introduced by the blending effect. Our results suggest
that although the majority of the M31 bulge are very old, the secular
evolutionary process still continuously builds up the M31 bulge slowly. We
compare our star formation history with an older analysis derived from the
spectral energy distribution fitting, which suggests that the latter one is
still a reasonable tool for the study of stellar populations in remote
galaxies.
