The Milky Way stellar halo out to 40 kpc : Squashed, broken but smooth

TL;DR

This study models the Milky Way's stellar halo using a new likelihood method applied to SDSS data, revealing a smooth, oblate halo with a broken power-law density profile and consistent flattening.

Contribution

Introduces a novel maximum likelihood approach to characterize the Milky Way stellar halo using extensive SDSS data, providing detailed density profile parameters.

Findings

Halo is smooth and oblate after masking overdensities.

Density follows a broken power-law with specific slopes and break radius.

Halo flattening remains constant with no evidence of triaxiality.

Abstract

We introduce a new maximum likelihood method to model the density profile of Blue Horizontal Branch and Blue Straggler stars and apply it to the Sloan Digital Sky Survey Data Release 8 (DR8) photometric catalogue. There are a large number (20,000) of these tracers available over an impressive 14,000 square degrees in both Northern and Southern Galactic hemispheres, and they provide a robust measurement of the shape of the Milky Way stellar halo. After masking out stars in the vicinity of the Virgo Overdensity and the Sagittarius stream, the data are consistent with a smooth, oblate stellar halo with a density that follows a broken power-law. The best fitting model has an inner slope = 2.3 and an outer slope = 4.6, together with a break radius occurring at 27 kpc and a constant halo flattening (that is, ratio of minor axis to major axis) of q = 0.6. Although a broken power-law describes the density fall-off most adequately, it is also well fit by an Einasto profile. There is no strong evidence for variations in flattening with radius, or for triaxiality of the stellar halo.