Constraining the assembly time of the stellar haloes of nearby Milky Way-mass galaxies through AGB populations

TL;DR

This study uses AGB star populations to estimate the assembly times of stellar haloes in Milky Way-mass galaxies, revealing that larger haloes tend to form stars more recently, which informs galaxy formation models.

Contribution

It introduces a novel method using AGB/RGB ratios to constrain stellar halo formation times in galaxies beyond the Local Group.

Findings

AGB stars help estimate halo formation times ($t_{90}$).

Stellar haloes of NGC 253 and NGC 891 formed stars relatively late (~6 Gyr ago).

Tentative anticorrelation between halo mass and age, with larger haloes forming more recently.

Abstract

The star formation histories (SFHs) of galactic stellar haloes offer crucial insights into the merger history of the galaxy and the effects of those mergers on their hosts. Such measurements have revealed that while the Milky Way's most important merger was 8-10 Gyr ago, M31's largest merger was more recent, within the last few Gyr. Unfortunately, the required halo SFH measurements are extremely observationally expensive outside of the Local Group. Here we use asymptotic giant branch (AGB) stars brighter than the tip of the red giant branch (RGB) to constrain stellar halo SFHs. Both stellar population models and archival datasets show that the AGB/RGB ratio constrains the time before which 90% of the stars formed, $t_{90}$. We find AGB stars in the haloes of three highly-inclined roughly Milky Way-mass galaxies with resolved star measurements from the Hubble Space Telescope; this population is most prominent in the stellar haloes of NGC 253 and NGC 891, suggesting that their stellar haloes contain stars born at relatively late times, with inferred $t_{90}\sim 6\pm1.5$Gyr. This ratio also varies from region to region, tending towards higher values along the major axis and in tidal streams or shells. By combining our measurements with previous constraints, we find a tentative anticorrelation between halo age and stellar halo mass, a trend that exists in models of galaxy formation but has never been elucidated before, i.e, the largest stellar haloes of Milky-Way mass galaxies were assembled more recently.