Mapping the Milky Way with Gaia Bp/Rp spectra II: The inner stellar halo traced by a large sample of blue horizontal branch stars

作者：Wenbo Wu, Xianhao Ye, C. Allende Prieto, Yuqin Chen, Xiang-Xiang Xue, Gang Zhao, Jingkun Zhao, David S. Aguado, J. I. Gónzalez Hernández, R. Rébolo · 发表于：arXiv (Cornell University) · 年份：2025 · DOI：10.48550/arxiv.2508.08784 · 研究领域：Stellar, planetary, and galactic studies、Astronomy and Astrophysical Research、Scientific Research and Discoveries

We selected BHB stars based on synthetic photometry and stellar atmosphere parameters inferred from Gaia Bp/Rp spectra. We generated the synthetic SDSS broad-band $ugr$ and Pristine narrow-band CaHK magnitudes from Gaia Bp/Rp data. A photometric selection of BHB candidates was made in the $(u-g, g-r)$ and $(u-\mathrm{CaHK},g-r)$ color-color spaces. A spectroscopic selection in $T_\mathrm{eff}-\log g$ space was applied to remove stars with high surface gravity. The selection function of BHB stars was obtained by using the Gaia DR3 photometry. A non-parametric method that allows the variation in the vertical flattening $q$ with the Galactic radius, was adopted to explore the density shape of the stellar halo. We present a catalog of 44,552 high latitude ($|b|>20^\circ$) BHB candidates chosen with a well-characterized selection function. The stellar halo traced by these BHB stars is more flattened at smaller radii ($q=0.4$ at $r\sim8$ kpc), and becomes nearly spherical at larger radii ($q=0.8$ at $r\sim25$ kpc). Assuming a variable flattening and excluding several obvious outliers that might be related to the halo substructures or contaminants, we obtain a smooth and consistent relationship between $r$ and $q$, and the density profile is best fit with by a single power law with an index $α=-4.65\pm0.04$.