# Mapping the Milky Way with LAMOST I: Method and overview

## TL;DR

This paper introduces a statistical method to accurately derive the Milky Way's stellar density profiles from spectroscopic survey data, accounting for selection effects, and applies it to LAMOST data to map the Galaxy's disk and halo.

## Contribution

The paper presents a novel approach to recover true stellar density profiles from spectroscopic surveys by modeling and correcting for complex selection functions.

## Key findings

- The Galactic disk extends to about 19 kpc with a smooth transition to the halo.
- No evidence of Monoceros ring over-density in the anti-center direction.
- The stellar halo is oblate within 20 kpc and nearly spherical beyond 30 kpc.

## Abstract

We present a statistical method to derive the stellar density profiles of the Milky Way from spectroscopic survey data, taking into account selection effects. We assume that the selection function of the spectroscopic survey is based on photometric colors and magnitudes and possibly altered during observations and data reductions. Then the underlying selection function for a line-of-sight 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 spectroscopically measured stellar density profile multiplied by the selection function. The method is validated using Galaxia mock data with two different selection functions. We demonstrate that the derived stellar density profiles 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 disk and halo, respectively, using the LAMOST RGB stars. The Galactic disk extends to about R=19 kpc, where the disk still contributes about 10% to the total stellar surface density. Beyond this radius, the disk smoothly transitions to the halo without any truncation, bending, or broken. Moreover, no over-density corresponding to the Monoceros ring is found in the Galactic anti-center direction. The disk shows moderate north-south asymmetry at radii larger than 12 kpc. On the other hand, the R-Z tomographic map directly shows that the stellar halo is substantially oblate within a Galactocentric radius of 20 kpc and gradually becomes nearly spherical beyond 30 kpc.