The Milky Way Tomography with Subaru Hyper Suprime-Cam. II. Global halo structure

TL;DR

This study models the Milky Way's stellar halo within 70 kpc using Subaru HSC data, revealing a double power-law density profile with implications for galaxy formation history.

Contribution

Developed a forward-modeling framework that accounts for uncertainties and survey limitations to accurately characterize the Milky Way's stellar halo structure.

Findings

The stellar halo follows a double power-law density profile.

Inner and outer slopes are approximately -3.3 and -4.8.

Break radius of the halo is about 17.4 kpc.

Abstract

We investigate the structure of the Milky Way's stellar halo within 70 kpc of the Sun using a wide-field photometric catalog obtained from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (HSC-SSP). We employ a large sample of main-sequence turn-off stars as distance tracers. To robustly derive the structural parameters of the stellar halo, we develop a forward-modeling framework that explicitly accounts for distance uncertainties, the solar position, and the limited sky coverage of the survey. Applying this method to the HSC-SSP catalog, we found that the smooth stellar halo is well described by a double power-law density profile, with inner and outer slope of approximately -3.3 and -4.8, respectively, with a break radius of 17.4 kpc. The outer steep density slope derived in this work supports a picture in which the present-day structure of the Milky Way's stellar halo is influenced by early massive accretion events, consistent with inferences from kinematic substructures such as Gaia Enceladus/Sausage. Ongoing wide-field imaging surveys, including UNIONS and LSST, will provide further constraints on the structure of the stellar halo and key insights into its formation history.