Characterising stellar halo populations I: An extended distribution function for halo K giants

TL;DR

This paper develops an extended distribution function model for K giant stars in the Milky Way's stellar halo, revealing detailed structure, metallicity gradients, and anisotropy variations with radius, enhancing understanding of halo composition and dynamics.

Contribution

It introduces a novel EDF that incorporates metallicity and actions, providing a comprehensive model of the stellar halo's structure and kinematics based on SEGUE data.

Findings

Halo density profile steepens with radius from -2 to -4

Halo's axis ratio increases from 0.7 to near 1 with radius

Metal-rich stars are more tightly confined and flattened

Abstract

We fit an Extended Distribution Function (EDF) to K giants in the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. These stars are detected to radii ~80 kpc and span a wide range in [Fe/H]. Our EDF, which depends on [Fe/H] in addition to actions, encodes the entanglement of metallicity with dynamics within the Galaxy's stellar halo. Our maximum-likelihood fit of the EDF to the data allows us to model the survey's selection function. The density profile of the K giants steepens with radius from a slope ~-2 to ~-4 at large radii. The halo's axis ratio increases with radius from 0.7 to almost unity. The metal-rich stars are more tightly confined in action space than the metal-poor stars and form a more flattened structure. A weak metallicity gradient ~-0.001 dex/kpc, a small gradient in the dispersion in [Fe/H] of ~0.001 dex/kpc, and a higher degree of radial anistropy in metal-richer stars result. Lognormal components with peaks at ~-1.5 and ~-2.3 are required to capture the overall metallicity distribution, suggestive of the existence of two populations of K giants. The spherical anisotropy parameter varies between 0.3 in the inner halo to isotropic in the outer halo. If the Sagittarius stream is included, a very similar model is found but with a stronger degree of radial anisotropy throughout.