Diverse Stellar Haloes in Nearby Milky Way-Mass Disc Galaxies

TL;DR

This study investigates the properties of stellar haloes in six nearby Milky Way-mass galaxies, revealing diverse structures, correlations with metallicity, and comparisons with cosmological models, enhancing understanding of galaxy formation.

Contribution

It provides detailed measurements of stellar halo density profiles, shapes, and metallicity correlations, and compares these observations with cosmological simulations, highlighting agreements and discrepancies.

Findings

Stellar halo density profiles follow power laws with slopes between -2 and -3.7.

Stellar halo masses range from 1 to 6 billion solar masses.

Strong correlation between stellar halo metallicity and mass.

Abstract

We have examined the resolved stellar populations at large galactocentric distances along the minor axis (from 10 kpc up to between 40 and 75 kpc), with limited major axis coverage, of six nearby highly-inclined Milky Way-mass disc galaxies using HST data from the GHOSTS survey. We select red giant branch stars to derive stellar halo density profiles. The projected minor axis density profiles can be approximated by power laws with projected slopes of between $-2$ and $-3.7$ and a diversity of stellar halo masses of $1-6\times 10^{9}M_{\odot}$, or $2-14\%$ of the total galaxy stellar masses. The typical intrinsic scatter around a smooth power law fit is $0.05-0.1$ dex owing to substructure. By comparing the minor and major axis profiles, we infer projected axis ratios $c/a$ at $\sim 25$ kpc between $0.4-0.75$. The GHOSTS stellar haloes are diverse, lying between the extremes charted out by the (rather atypical) haloes of the Milky Way and M31. We find a strong correlation between the stellar halo metallicities and the stellar halo masses. We compare our results with cosmological models, finding good agreement between our observations and accretion-only models where the stellar haloes are formed by the disruption of dwarf satellites. In particular, the strong observed correlation between stellar halo metallicity and mass is naturally reproduced. Low-resolution hydrodynamical models have unrealistically high stellar halo masses. Current high-resolution hydrodynamical models appear to predict stellar halo masses somewhat higher than observed but with reasonable metallicities, metallicity gradients and density profiles.