Slicing and dicing the Milky Way disk in SDSS

TL;DR

This study analyzes the kinematics of Milky Way disk stars using SDSS data, developing methods to correct halo contamination and modeling the velocity ellipsoid to understand disk dynamics and mass distribution.

Contribution

It introduces a new method for correcting halo contamination and models the velocity ellipsoid as a function of height and metallicity in the Galactic disk.

Findings

Metal-poor stars rotate slower than metal-rich stars.

Disk dominates the circular speed near the Sun.

Estimated local surface mass density is around 66 M/pc^2.

Abstract

We use the Stripe 82 proper motion catalogue of Bramich et al. (2008) to study the kinematics of Galactic disk stars in the solar neighborhood. We select samples of dwarf stars with reliable spectra and proper motions. They have cylindrical polar radius between 7 < R < 9 kpc, heights from the Galactic plane satisfying z < 2 kpc and span a range of metallicities -1.5 < [Fe/H] < 0. We develop a method for calculating and correcting for the halo contamination in our sample using the distribution of rotational velocities. Two Gaussians representing disk and halo populations are used to fit the radial and vertical velocity distributions via maximum likelihood methods. For the azimuthal velocities the same technique is used, except that a skewed non-Gaussian functional form now represents the disk velocity distribution. This enables us to compute the dispersions and cross-terms (the tilt and the vertex deviation) of the velocity ellipsoid as a function of height and metallicity. We also investigate the rotation lag of the disk, finding that the more metal-poor stars rotate significantly slower than the metal-rich stars. These samples provide important constraints on heating mechanisms in the Galactic disk and can be used for a variety of applications. We present one such application, employing the Jeans equations to provide a simple model of the potential close to the disk. Our model is in excellent agreement with others in the literature and provides an indication the disk, rather than the halo, dominates the circular speed at the solar neighborhood. We obtain a surface mass density within 1.1 kpc of around 66 M/pc^2 and estimate a local halo density of 0.015 M/pc^3 = 0.57 GeV/cm^3.