The Outer Halo of the Milky Way as Probed by RR Lyr Variables from the Palomar Transient Facility

TL;DR

This study uses RR Lyrae stars from the Palomar Transient Facility to map the Milky Way's outer halo, revealing its mass distribution and dark matter content through radial velocity measurements and stellar density analysis.

Contribution

It presents a new sample of 112 RR Lyrae stars beyond 50 kpc, identified via machine learning, and provides the first detailed velocity dispersion profile of the Milky Way's outer halo.

Findings

Radial velocity dispersion decreases from ~90 km/s at 50 kpc to ~65 km/s near 100 kpc.

Stellar density in the outer halo declines as r to the power of -4.

Four RR Lyrae stars are found beyond 100 kpc.

Abstract

RR Lyr stars are ideal massless tracers that can be used to study the total mass and dark matter content of the outer halo of the Milky Way. This is because they are easy to find in the light curve databases of large stellar surveys and their distances can be determined with only knowledge of the light curve. We present here a sample of 112 RR Lyr beyond 50 kpc in the outer halo of the Milky Way, excluding the Sgr streams, for which we have obtained moderate resolution spectra with Deimos on the Keck 2 Telescope. Four of these have distances exceeding 100 kpc. These were selected from a much larger set of 447 candidate RR Lyr which were datamined using machine learning techniques applied to the light curves of variable stars in the Palomar Transient Facility database. The observed radial velocities taken at the phase of the variable corresponding to the time of observation were converted to systemic radial velocities in the Galactic standard of rest. From our sample of 112 RR Lyr we determine the radial velocity dispersion in the outer halo of the Milky Way to be ~90 km/s at 50 kpc falling to about 65 km/s near 100 kpc once a small number of major outliers are removed. With reasonable estimates of the completeness of our sample of 447 candidates and assuming a spherical halo, we find that the stellar density in the outer halo declines as the -4 power of r.