We explore the assembly history of the M31 bulge within a projected major-axis radius of 180″ ( \sim 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 , \sim Z _ { \odot } ) 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 \sim 1 % of the total stellar mass in the bulge .
These stars could be related to the secular growth of the M31 bulge .