# Mapping the Milky Way with LAMOST II: the stellar halo

## TL;DR

This study measures the density and shape of the Milky Way's stellar halo using LAMOST K giants, revealing a single power-law profile and radius-dependent flattening, along with identifying tidal streams and an unknown overdensity.

## Contribution

It provides a model-independent measurement of the stellar halo's density profile and flattening variation with radius using a large spectroscopic dataset.

## Key findings

- Halo density follows a power law with index ~5.
- Halo flattening varies from 0.64 to 0.96 with radius.
- Identifies tidal streams and an unknown overdensity.

## Abstract

The radial number density and flattening of the Milky Way's stellar halo is measured with $\mathrm{5351}$ metal-poor ([Fe/H]$<-1$) K giants from LAMOST DR3, using a nonparametric method which is model independent and largely avoids the influence of halo substucture. The number density profile is well described by a single power law with index $5.03^{+0.64}_{-0.64}$, and flattening that varies with radius. The stellar halo traced by LAMOST K giants is more flattened at smaller radii, and becomes nearly spherical at larger radii. The flattening, $q$, is about 0.64, 0.8, 0.96 at $r=15$, 20 and 30 kpc (where $r=\sqrt{R^2+\left[Z/q\left(r\right)\right]^2}$), respectively. Moreover, the leading arm of the Sagittarius dwarf galaxy tidal stream in the north, and the trailing arm in the south, are significant in the residual map of density distribution. In addition, an unknown overdensity is identified in the residual map at (R,Z)=(30,15) kpc.