We present the study of stellar populations in the central 5.5 arcmin (˜1.2 kpc) of the M31 bulge by using the optical colour magnitude diagram derived from the observations taken by HST Advanced Camera for Surveys 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 colour magnitude diagram fitting is performed for each of them. We confirm that the majority of stars (>70 per cent) 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 arcsec. Through simulation, we find that these intermediate-age stars cannot be the artefacts 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.
The Star Formation History in the M31 Bulge