Aims: We derive the morphology of the stellar component in the outer halo volume, and search for possible overdensities due to substructures therein.
Methods: We made use of some of the data releases of the spectroscopic survey LAMOST DR8-DR9 in tandem with distance determinations for two subsamples, that is, of K-giants and M-giants, respectively, making up 60 000 stars. These distance are obtained through Bayesian techniques that derive absolute magnitudes as a function of measured spectroscopic parameters. Our calculation of the density from these catalogues requires: (1) derivation of the selection function; and (2) a correction for the convolution of the distance errors, which we carried out with Lucy's inversion of the corresponding integral equation.
Results: The stellar density distribution of the outer halo (distance to the Galactic centre, rG, of between 25 and 90 kpc) is a smooth monotonously decreasing function with a dependence of approximately ρ ∝ rG−n, with n = 4.6 ± 0.4 for K-giants and n = 4.5 ± 0.2 for M-giants, and with a insignificant oblateness. The value of n is independent of the angular distance to the Sagittarius tidal stream plane, which is what would be expected if such a stream did not exist in the anticenter positions or had a negligible imprint in the density distribution in the outer halo. Apart from random fluctuations or minor anomalies in some lines of sight, we do not see substructures superimposed in the outer halo volume within the resolution that we are using and limited by the error bars. This constrains the mass of over- and under-densities in the outer halo to be of ≲103 M⊙ deg−2, whereas the total mass of the stellar halo, including inner and outer parts, is ∼7 × 108 M⊙.