@article{f46b74bc5d8f4ebc83e751914e6550f9,
title = "The stellar halo of the Milky Way traced by blue horizontal-branch stars in the Subaru Hyper Suprime-Cam Survey",
abstract = "We report on the global structure of the Milky Way (MW) stellar halo up to its outer boundary based on the analysis of blue horizontal-branch stars (BHBs). These halo tracers are extracted from the (g, r, i, z)-band multi-photometry in the internal data release of the ongoing Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) surveyed over a ∼550 deg2 area. In order to select the most likely BHBs by removing blue straggler stars (BSs) and other contamination in a statistically significant manner, we have developed and applied an extensive Bayesian method, instead of the simple color cuts adopted in our previous work, where each of the template BHBs and non-BHBs obtained from the available catalogs are represented as a mixture of multiple Gaussian distributions in the color-color diagrams. We found from the candidate BHBs in the range of 18.5 < g < 23.5 mag that the radial density distribution over a Galactocentric radius of r = 36-360 kpc can be approximated as a single power-law profile with an index of α =3.74+0.21-0.22 or a broken power-law profile with an index of αin=2.92+0.33-0.33 at r below a broken radius of b=160+18-19kpc and a very steep slope of αout=15.0+3.7-4.5 at r > rb. The latter profile with a prolate shape having an axial ratio of q=1.72+0.44-0.28 is most likely and this halo may hold a rather sharp boundary at r 160 kpc. The slopes of the halo density profiles are compared with those from the suite of hydrodynamical simulations for the formation of stellar halos. This comparison suggests that the MW stellar halo may consist of the two overlapping components: the in situ inner halo as probed by RR Lyrae stars showing a relatively steep radial density profile and the ex situ outer halo with a shallow profile probed by BHBs here, which is made by accretion of small stellar systems.",
keywords = "Galaxy: halo, Galaxy: structure, stars: horizontal-branch",
author = "Tetsuya Fukushima and Masashi Chiba and Mikito Tanaka and Kohei Hayashi and Daisuke Homma and Sakurako Okamoto and Yutaka Komiyama and Masayuki Tanaka and Nobuo Arimoto and Tadafumi Matsuno",
note = "Funding Information: This work is supported in part by JSPS Grant-in-Aid for Scientific Research (B) (No. 25287062) and MEXT Grant-in-Aid for Scientific Research (No. 16H01086, 17H01101 and 18H04334 for MC, No. 18H04359 and No. 18J00277 for KH). TM is supported by Grant-in-Aid for JSPS Fellows (No. 18J11326). Funding Information: The Hyper Suprime-Cam (HSC) collaboration includes the astronomical communities of Japan and Taiwan, and Princeton University. The HSC instrumentation and software were developed by the National Astronomical Observatory of Japan (NAOJ), the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), the University of Tokyo, the High Energy Accelerator Research Organization (KEK), the Academia Sinica Institute for Astronomy and Astrophysics in Taiwan (ASIAA), and Princeton University. Funding was contributed by the FIRST program from Japanese Cabinet Office, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Society for the Promotion of Science (JSPS), Japan Science and Technology Agency (JST), the Toray Science Foundation, NAOJ, Kavli IPMU, KEK, ASIAA, and Princeton University. This paper makes use of software developed for the Large Synoptic Survey Telescope. We thank the LSST Project for making their code freely available. The Pan-STARRS1 (PS1) Surveys have been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy and the Max Planck Institute for Extraterrestrial Physics, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen{\textquoteright}s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation under Grant No.AST-1238877, the University of Maryland, and Eotvos Lorand University (ELTE). Publisher Copyright: {\textcopyright} 2019 The Author(s). Published by Oxford University Press on behalf of the Astronomical Society of Japan.",
year = "2019",
month = aug,
day = "1",
doi = "10.1093/pasj/psz052",
language = "English",
volume = "71",
journal = "Publications of the Astronomical Society of Japan",
issn = "0004-6264",
publisher = "Astronomical Society of Japan",
number = "4",
}