Authors:
Dong H., Li Z., Daniel Wang Q., Lauer T.R., Olsen K.A.G., Saha A., Dalcanton J.J., Williams B.F.
Journal:
Monthly Notices of the Royal Astronomical Society
Abstract:
We explore the assembly history of the M31 bulge within a projected major-axis radius of 180 arcsec (~680 pc) by studying its stellar populations in Hubble Space TelescopeWide Field Camera 3 and Advanced Camera for Surveys observations. Colours formed by comparing nearultraviolet versus 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, ~Z<inf>⊙</inf>) 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 per cent at 680 pc away from the centre. We exclude contamination from the M31 disc and/or halo as the main origin for this population. Our results thus suggest that intermediate-age stars exist beyond the central 5 arcsec (19 pc) of M31 and contribute ~1 per cent of the total stellar mass in the bulge. These stars could be related to the secular growth of the M31 bulge. © 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
URL:
https://ui.adsabs.harvard.edu/#abs/2015MNRAS.451.4126D/abstract
Keywords:
Galaxies: abundances; Galaxies: bulges; Galaxies: evolution; Galaxies: stellar content