Age constraints for an M31 globular cluster from main-sequence photometry

We present a color-magnitude diagram (CMD) of the globular cluster SKHB 312 in the Andromeda galaxy (M31), obtained with the Advanced Camera for Surveys on the Hubble Space Telescope. The cluster was included in deep observations taken to measure the star formation history of the M31 halo. Overcoming a very crowded field, our photometry of SKHB 312 reaches m(V) approximate to 30.5 mag, more than 1 mag below the main-sequence turnoff. These are the first observations to allow a direct age estimate from the turnoff in an old M31 cluster. We analyze its CMD and luminosity function using a finely spaced grid of isochrones that have been calibrated using observations of Galactic clusters taken with the same camera and filters. The luminosity difference between the subgiant and horizontal branches is approximate to0.2 mag smaller in SKHB 312 than in the Galactic clusters 47 Tuc and NGC 5927, implying that SKHB 312 is 2-3 Gyr younger. A quantitative comparison to isochrones yields an age of 10(-1)(+2.5) Gyr.