Velocity Dispersion Profile of the Milky Way Halo

TL;DR

This study measures the velocity dispersion profile of the Milky Way halo using a large sample of distant stars, providing insights into the galaxy's mass distribution and confirming previous findings with new data and methods.

Contribution

It presents the largest sample of distant halo stars to date and applies both parametric and non-parametric methods to derive the velocity dispersion profile.

Findings

Velocity dispersion declines with radius, at -0.38 km/s/kpc between 15 and 75 kpc.

Sample contains twice as many stars beyond 50 kpc compared to previous surveys.

Results are consistent with earlier studies, supporting models of the Milky Way's mass distribution.

Abstract

We present a spectroscopic sample of 910 distant halo stars from the Hypervelocity Star survey from which we derive the velocity dispersion profile of the Milky Way halo. The sample is a mix of 74% evolved horizontal branch stars and 26% blue stragglers. We estimate distances to the stars using observed colors, metallicities, and stellar evolution tracks. Our sample contains twice as many objects with R>50 kpc as previous surveys. We compute the velocity dispersion profile in two ways: with a parametric method based on a Milky Way potential model, and with a non-parametric method based on the caustic technique originally developed to measure galaxy cluster mass profiles. The resulting velocity dispersion profiles are remarkably consistent with those found by two independent surveys based on other stellar populations: the Milky Way halo exhibits a mean decline in radial velocity dispersion of -0.38+-0.12 km/s/kpc over 15<R<75 kpc. This measurement is a useful basis for calculating the total mass and mass distribution of the Milky Way halo.