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 \pm 2.0 \times 10 ^ { 8 } L _ { \odot } excluding Sgr , and L _ { halo } = 9.4 \pm 2.4 \times 10 ^ { 8 } L _ { \odot } 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 ^ { \star } / L = 1.5 ) , we estimate a stellar halo mass of M ^ { \star } _ { halo } = 1.4 \pm 0.4 \times 10 ^ { 9 } M _ { \odot } .
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 \sim 10 ^ { 9 } M _ { \odot } mass and an average metallicity of \langle \mathrm { [ Fe / H ] } \rangle \sim - 1.5 for the Galactic halo points to an ancient ( \sim 10 Gyr ) merger event .