Stellar halo density with LAMOST K and M giants

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: martin@lopez-corredoira.com
² PIFI-Visiting Scientist 2023 of Chinese Academy of Sciences at Purple Mountain Observatory, Nanjing 210023, and National Astronomical Observatories, Beijing 100012, PR China
³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁴ School of Physics and Astronomy, China West Normal University, 1 ShiDa Road, Nanchong 637002, PR China
⁵ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, PR China
⁶ Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁷ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁸ University of Chinese Academy of Sciences, Beijing 100049, PR China

Received: 29 November 2023
Accepted: 5 February 2024

Abstract

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, r_G, of between 25 and 90 kpc) is a smooth monotonously decreasing function with a dependence of approximately ρ ∝ r_G⁻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_⊙.