The Stellar Density Profile of the Distant Galactic Halo

TL;DR

This study uses extensive photometry to map the Milky Way's stellar halo, revealing a consistent r^{-3.5} density profile from 30 to 80 kpc and highlighting the influence of the Sagittarius stream on shape measurements.

Contribution

It provides the first large-scale, unbiased measurement of the stellar halo's density profile out to 80 kpc, improving understanding of the halo's structure and formation history.

Findings

Halo maintains a r^{-3.5} profile from 30 to 80 kpc

No evidence of truncation or sharp break in the profile

Sagittarius stream dominates shape measurements

Abstract

We use extensive gravity-sensitive DDO 51 photometry over 5100 square degrees, combined with SDSS broadband photometry, to select a catalog of $\sim 4,000$ giant stars covering a large fraction of the high Galactic latitude sky and reaching out to $\sim 80$ kpc in the Galactic halo. This sample of bright and unbiased tracers enables us to measure the radial profile and 3D structure of the stellar halo to large distance which had previously only been measured with sparse tracers or small samples. Using population synthesis models to reproduce the observed giant star luminosity function, we find that the halo maintains a $r^{-3.5}$ profile from $30$ to $80$ kpc with no signs of a truncation or sharp break over this range. The radial profile measurement is largely insensitive to individual halo substructure components, but we find that attempting to measure the shape of the halo is overwhelmed by the Sagittarius stream such that no ellipsoidal shape is a satisfactory description in this region. These measurements allow us to begin placing the Milky Way in context with the growing sample of external galaxies where similar halo profile measurements are available, with the goal of further linking the properties of stellar halos to the accretion histories that formed them.