The stellar density of the Milky Way with LAMOST RGB stars

Title: The stellar density of the Milky Way with LAMOST RGB stars

Abstract: 
We present a statistical method to derive the stellar density profiles of the Milky Way from spectroscopic survey data taking into account the selection effect. We assume that the selection function of the spectroscopic survey is only in color-magnitude diagram. If no other quantities, e. g., metallicity, proper motion, and age etc., are get involved in the spectroscopic targeting, the underlying selection function can be well recovered by comparing the distribution of the spectroscopic stars in a color-magnitude plane with that of the photometric dataset. Subsequently, the stellar density profile along a line-of-sight can be derived from the spectroscopic stellar density profile multiplied by the selection function. The method is then validated using the Galaxia mock data. We demonstrate that the spectroscopically derived stellar density profiles can well reconstruct the true ones not only for the full targets, but also for the sub-populations selected from the full dataset. Finally, the method is applied to map the density profiles for the Galactic halo and disk, respectively, using the LAMOST RGB stars. The stellar halo is substantially oblate within 20 kpc and become relatively spherical beyond 30 kpc. On the other hand, the Galactic disk is found to extend to beyond 20\,kpc. The perfectly exponential stellar surface density profile smoothly transits to the halo at about 25 kpc without any truncation, bending, or breaking.