Constraints on stellar halo ages using AGB/RGB star ratios for Milky Way-mass galaxies

Stellar halos contain the debris from the disruption (or accretion) of satellite galaxies. Previous work from the GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick disks, and Star clusters) survey shows that stellar halos show a diversity in their masses, metallicities, shapes, and degrees of substructure. Comparison with galaxy formation models show that this diversity is most naturally reproduced by the largest satellite to have been disrupted by the main galaxy - larger satellites disrupt into more massive, more metal-rich halos and dominate the stellar populations. If one could measure the ages of the stellar populations in halos, one could place constraints on when this most massive accretion event happened. Unfortunately, most tracers of stellar population age are very faint and are observationally unavailable for most galaxies.

Here we discuss the constraints on stellar halo ages that can be placed using bright AGB (Asymptotic Giant Branch) stars. AGB stars are particularly numerous for intermediate age (~Gyr to several Gyr old) populations. We focus on three Milky Way-mass galaxies (M81, NGC 253, and NGC 891) that have well-measured AGB stars in their halos within galactocentric radii of ~20kpc. From the observations we determine an overall ratio of AGB to RGB stars. The AGB/RGB ratios range from roughly -0.8 to -1.2 dex, in excess of the values expected for ancient >10Gyr old stellar populations. We compare the ratios with stellar population models and observations for nearby dwarf galaxies, allowing us to place broad constraints on when star formation shut off, and thereby constrain the merger time. While M81 and NGC 253 have minor axis ratios suggesting relatively old accretions, NGC 891's ratios suggest that its stellar halo is younger. With previous observations of tidal streams in NGC 891's stellar halo, it appears that most of NGC 891's stellar halo material was delivered in the last few Gyr by the disruption of a large satellite galaxy. Looking forward to the era of the James Webb and Nancy Grace Roman Space Telescopes, we suggest that the ratio of AGB/RGB stars may be an observationally-accessible, if rough, probe of stellar halo age and thereby measure of the time of the largest merger experienced by a galaxy.