Characterizing stellar halo populations - I. An extended distribution function for halo K giants
Abstract
We fit an extended distribution function (EDF) to K giants in the Sloan Extension for Galactic Understanding and Exploration 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-1, a small gradient in the dispersion in [Fe/H] of ∼0.001 dex kpc-1, and a higher degree of radial anisotropy 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- August 2016
- DOI:
- 10.1093/mnras/stw744
- arXiv:
- arXiv:1603.09332
- Bibcode:
- 2016MNRAS.460.1725D
- Keywords:
-
- methods: data analysis;
- Galaxy: halo;
- Galaxy: kinematics and dynamics;
- Galaxy: stellar content;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 15 pages, 15 figures, accepted for publication in MNRAS