Photometric Evidence of an Intermediate-age Stellar Population in the Inner Bulge of M31

TL;DR

This study provides photometric evidence for an intermediate-age stellar population in the inner bulge of M31, indicating recent star formation activity beyond the central region and contributing to the understanding of bulge assembly.

Contribution

It is the first to identify and characterize a metal-rich, intermediate-age stellar population in M31's bulge using HST observations, revealing a radial gradient in age and mass fraction.

Findings

Intermediate-age stars are present beyond 19 pc from M31's center.

These stars constitute about 1% of the bulge's stellar mass.

The intermediate-age population shows an increase in age and mass fraction with radius.

Abstract

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.