Structure of the Milky Way stellar halo out to its outer boundary with blue horizontal-branch stars

TL;DR

This study maps the Milky Way's stellar halo beyond 50 kpc using blue horizontal-branch stars from the HSC-SSP survey, revealing substructures and a complex, possibly disturbed, prolate shape with a steep density profile.

Contribution

It provides the first detailed analysis of the outer stellar halo structure beyond 50 kpc using HSC-SSP data, identifying substructures and characterizing the density profile and shape.

Findings

Halo follows a power-law density profile with index ~3.3 to 3.5.

Detected significant substructures in the halo, especially in specific fields.

Halo may be prolate and disturbed, possibly linked to the Sagittarius stream.

Abstract

We present the structure of the Milky Way stellar halo beyond Galactocentric distances of $r = 50$ kpc traced by blue horizontal-branch (BHB) stars, which are extracted from the survey data in the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We select BHB candidates based on $(g,r,i,z)$ photometry, where the $z$-band is on the Paschen series and the colors that involve the $z$-band are sensitive to surface gravity. About 450 BHB candidates are identified between $r = 50$ kpc and 300 kpc, most of which are beyond the reach of previous large surveys including the Sloan Digital Sky Survey. We find that the global structure of the stellar halo in this range has substructures, which are especially remarkable in the GAMA15H and XMM-LSS fields in the HSC-SSP. We find that the stellar halo can be fitted to a single power-law density profile with an index of $\alpha \simeq 3.3$ ($3.5$) with (without) these fields and its global axial ratio is $q \simeq 2.2$ ($1.3$). Thus, the stellar halo may be significantly disturbed and be made in a prolate form by halo substructures, perhaps associated with the Sagittarius stream in its extension beyond $r \sim 100$ kpc. For a broken power-law model allowing different power-law indices inside/outside a break radius, we obtain a steep power-law slope of $\alpha \sim 5$ outside a break radius of $\simeq 100$ kpc ($200$ kpc) for the case with (without) GAMA15H and XMM-LSS. This radius of $200$ kpc might be as close as a halo boundary if there is any, although larger BHB sample is required from further HSC-SSP survey to increase its statistical significance.