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 on-going Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) surveyed over $\sim 550$~deg${}^2$ area. In order to select 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 is 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 $\alpha =3.74_{-0.22}^{+0.21}$ or a broken power-law profile with an index of ${\alpha }_{\mathrm{in}}=2.92_{-0.33}^{+0.33}$ at $r$ below a broken radius of $r_{\mathrm{b}}=160_{-19}^{+18}$~kpc and a very steep slope of ${\alpha }_{\mathrm{out}}=15.0_{-4.5}^{+3.7}$ at $r>r_{\mathrm{b}}$. The latter profile with a prolate shape having an axial ratio of $q=1.72_{-0.28}^{+0.44}$ is most likely and this halo may hold a rather sharp boundary at r=160kpc. 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.

15 pages, 10 figures, 3 tables, version to be published in PASJ

References in corpus (16)

• Introducing the Illustris Project: Simulating the coevolution of dark and visible matter in the Universe
• Properties of galaxies reproduced by a hydrodynamic simulation
• Introducing the Illustris Project: the evolution of galaxy populations across cosmic time
• The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
• A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy
• New white dwarf stars in the Sloan Digital Sky Survey Data Release 10
• The stellar halo of the Galaxy
• Halo Mass and Assembly History Exposed in the Faint Outskirts: the Stellar and Dark Matter Haloes of Illustris Galaxies
• Touching The Void: A Striking Drop in Stellar Halo Density Beyond 50 kpc
• Revealing Substructure in the Galactic Halo - The SEKBO RR Lyrae Survey
• A New Milky Way Satellite Discovered In The Subaru/Hyper Suprime-Cam Survey
• Mapping the Milky Way with LAMOST II: the stellar halo
• The Outer Halo of the Milky Way as Probed by RR Lyr Variables from the Palomar Transient Facility
• To the Galactic Virial Radius with Hyper Suprime-Cam
• The Stellar Density Profile of the Distant Galactic Halo
• Disentangling the Virgo Overdensity with RR Lyrae stars

Cited by in corpus (15)

• The ARTEMIS simulations: stellar haloes of Milky Way-mass galaxies
• The mass of the Milky Way out to 100 kpc using halo stars
• Quantifying the Stellar Halo's Response to the LMC's Infall with Spherical Harmonics
• Mapping the anisotropic Galactic stellar halo with Blue Horizontal Branch stars
• Stellar halos from The Dragonfly Edge-on Galaxies Survey
• The Cetus-Palca stream: A disrupted small dwarf galaxy. A prequel to the science possible with WEAVE with precise spectro-photometric distances
• A LAMOST BHB Catalog and Kinematics Therein I: Catalog and Halo Properties
• The Photo-Astrometric Vertical Tracer Density of the Milky Way II: Results from Gaia
• Observing the stellar halo of Andromeda in cosmological simulations: the Auriga2PAndAS pipeline
• The Photo-Astrometric Vertical Tracer Density of the Milky Way I: The Method
• A Pristine-UNIONS view on the Galaxy: Kinematics of the distant spur feature of the Sagittarius stream traced by Blue Horizontal Branch stars
• Mapping the Milky Way's stellar halo with 2D data
• Analogy of space-time as an elastic medium, estimation creep coefficient of space from MOND theory, gravitational lensing and via time data from the GPS effect, discussion of the results for dark matter and Einstein's field equation
• H$\alpha$ emission in the outskirts of galaxies at z=0.4
• Dwarfs or giants? Stellar metallicities and distances in the Canada-France-Imaging-Survey from $ugrizG$ multi-band photometry