Photometric Evidence of an Intermediate-age Stellar Population in the
  Inner Bulge of M31
  We explore the assembly history of the M31 bulge within a projected
major-axis radius of 180" (~680 pc) by studying its stellar populations in
Hubble Space Telescope WFC3 and ACS observations. Colors formed by comparing
near-ultraviolet vs. optical bands are found to become bluer with increasing
major-axis radius, which is opposite to that predicted if the sole sources of
near-ultraviolet light were old extreme horizontal branch stars with a negative
radial gradient in metallicity. Spectral energy distribution fits require a
metal-rich intermediate-age stellar population (300 Myr to 1 Gyr old, ~solar
metallicity) in addition to the dominant old population. The radial gradients
in age and metallicity of the old stellar population are consistent with those
in previous works. For the intermediate-age population, we find an increase in
age with radius and a mass fraction that increases up to 2% at 680 pc away from
the center. We exclude contamination from the M31 disk and/or halo as the main
origin for this population. Our results thus suggest that intermediate-age
stars exist beyond the central 5" (19 pc) of M31 and contribute ~1% of the
total stellar mass in the bulge. These stars could be related to the secular
growth of the M31 bulge.
