The stellar halo of the Milky Way traced by blue horizontal-branch stars in the Subaru Hyper Suprime-Cam Survey (1904.04966v2)

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 $\sim550$~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 $\alpha=3.74^{+0.21}_{-0.22}$ or a broken power-law profile with an index of $\alpha_{\rm in}=2.92^{+0.33}_{-0.33}$ at $r$ below a broken radius of $r_{\rm b}=160^{+18}_{-19}$~kpc and a very steep slope of $\alpha_{\rm out}=15.0^{+3.7}_{-4.5}$ at $r>r_{\rm b}$. 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=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.

• The paper introduces a Bayesian method that accurately distinguishes blue horizontal-branch stars from similar contaminants to map the Milky Way’s halo.
• It leverages Subaru HSC survey data over ~550 deg² to reveal a broken power-law density profile with an inner slope of ≈2.92 and an outer slope of ≈15.0.
• The study finds the halo exhibits a prolate shape (axial ratio q ≈ 1.72), supporting a two-component formation history with both in situ and ex situ origins.

Analysis of the Milky Way Stellar Halo Using Blue Horizontal-Branch Stars in the Hyper Suprime-Cam Survey

The research presented in the paper "The stellar halo of the Milky Way traced by blue horizontal-branch stars in the Subaru Hyper Suprime-Cam Survey" delivers a detailed investigation into the structure of our galaxy's stellar halo by leveraging a significant data set obtained from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). This paper focuses on using Blue Horizontal-Branch (BHB) stars as primary tracers, based on their precise photometric properties in the $g$, $r$, $i$, and $z$ bands, processed through an innovative Bayesian framework. The application of an extensive Bayesian method marks a critical advancement from prior work, minimizing contamination effects from non-BHB stars, particularly Blue Straggler (BS) stars.

Methodology and Data Processing

The paper utilized the HSC-SSP Wide survey data encompassing $\sim550$ deg², meticulously processed to derive a robust catalog of point sources. The team implemented a Bayesian statistical approach to distinguish BHBs from similar sources such as BSs, white dwarfs (WDs), quasars (QSOs), and faint galaxies. The stellar classifications relied on multi-dimensional Gaussian mixtures that mapped the characteristic color distributions in the photometric space. This method significantly improves precision in isolating genuine BHB stars from contaminants that share similar photometric signatures, especially critical for faint data where contamination increases.

Results and Analysis

The stellar halo of the Milky Way is examined over a substantial radial distance from 36 kpc to 360 kpc in Galactocentric coordinates. The halo's density profile was characterized using several models, with the broken power-law model fitting the data most effectively. This model defines a relatively shallow slope, $\alpha_{\rm in} = 2.92$, within approximately 160 kpc and a starkly steep slope, $\alpha_{\rm out} = 15.0$, beyond this distance, suggesting a sharp outer boundary of the halo. Additionally, the analysis reveals the halo's prolate nature with an axial ratio $q = 1.72$. The research reports that this spatial distribution aligns with predictions from recent hydrodynamic simulations indicating the presence of both {\it in situ} and {\it ex situ} components of the halo, the latter being the result of accreted external systems.

Implications and Future Work

This work significantly refines our understanding of the Milky Way halo's structure, particularly in the outer regions, contributing valuable insights into the galaxy's formation and merging history. The paper exemplifies methodological improvements—especially the Bayesian methodology—to efficiently utilize photometric data for halo characterization. Future work could build upon these insights by integrating deeper photometric surveys and leveraging additional types of stellar tracers, such as RR Lyrae stars, to further refine the model of the galaxy's halo and enhance our comprehension of its evolutionary history.

The results highlight the galaxy's accretion history, suggesting a two-component halo with distinct structural properties. To gain a more holistic comprehension of the halo composition, future investigations could explore the combined analysis of different halo tracer populations across diverse datasets, ideally with encompassing simulations incorporating both kinematic and metallicity data.

In summary, this paper delivers an essential step in elucidating the large-scale structure of the Milky Way's halo, offering a methodological framework potentially extendable to other galactic systems. Further calibration of these techniques could enhance the precision of our cosmological models and our understanding of galaxy formation processes in various environments.