# The total stellar halo mass of the Milky Way

## TL;DR

This paper measures the Milky Way's stellar halo luminosity and mass using Gaia data, revealing a larger halo mass than previous estimates and suggesting an ancient merger event shaped its formation.

## Contribution

The study introduces a new method to quantify the Milky Way's stellar halo luminosity and mass using Gaia data and proper motion decomposition, providing updated estimates and insights into its formation history.

## Key findings

- Total stellar halo luminosity (excluding Sgr): (7.9 +/- 2.0) x 10^8 L_Sun
- Estimated stellar halo mass: (1.4 +/- 0.4) x 10^9 M_Sun
- Halo likely resulted from a ~10 Gyr old merger event

## Abstract

We measure the total stellar halo luminosity using red giant branch (RGB) stars selected from Gaia data release 2. Using slices in magnitude, colour and location on the sky, we decompose RGB stars belonging to the disc and halo by fitting 2-dimensional Gaussians to the Galactic proper motion distributions. The number counts of RGB stars are converted to total stellar halo luminosity using a suite of isochrones weighted by age and metallicity, and by applying a volume correction based on the stellar halo density profile. Our method is tested and calibrated using Galaxia and N-body models. We find a total luminosity (out to 100 kpc) of L_halo = 7.9 +/- 2.0 x 10^8 L_Sun excluding Sgr, and L_halo = 9.4 +/- 2.4 x 10^8 L_Sun including Sgr. These values are appropriate for our adopted stellar halo density profile and metallicity distribution, but additional systematics related to these assumptions are quantified and discussed. Assuming a stellar mass-to-light ratio appropriate for a Kroupa initial mass function (M*/L = 1.5), we estimate a stellar halo mass of M*_halo = 1.4 +/- 0.4 x 10^9 M_Sun. This mass is larger than previous estimates in the literature, but is in good agreement with the emerging picture that the (inner) stellar halo is dominated by one massive dwarf progenitor. Finally, we argue that the combination of a ~10^9 M_Sun mass and an average metallicity of <[Fe/H]> ~ -1.5 for the Galactic halo points to an ancient (~10 Gyr) merger event.