Mapping the Milky Way with Gaia XP spectra. II.

A photometric selection of BHB candidates is made in the (u-g, g-r) and (u-CaHK, g-r) color-color spaces. A spectroscopic selection in Teff-logg space is applied to remove stars with high surface gravity. The selection function of BHB stars is obtained by using the Gaia DR3 photometry.

A non-parametric method, which allows the variation of the vertical flattening q with the Galactic radius, is adopted to explore the density shape of the stellar halo. We construct a full-sky BHB catalog from Gaia Bp/Rp spectra and use it to explore the shape of the inner stellar halo. We present a catalog of 44552 high latitude (|b|>20{deg}) 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~8kpc), and becomes nearly spherical at larger radii (q=0.8 at r~25kpc). 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 fitted with by a single power law with an index alpha=-4.65+/-0.04.