The vertical velocity dispersion profile of the Galactic thick disk

TL;DR

This study measures the vertical velocity dispersion of thick disk stars in the Milky Way, finding unexpectedly low values and a small gradient, challenging previous assumptions about the thick disk's kinematic properties.

Contribution

It provides new measurements of the vertical velocity dispersion profile of the Galactic thick disk using a large sample of red giants, revealing lower dispersion values than previously reported.

Findings

Vertical velocity dispersion (sigma_W) is about 30 km/s at 2 kpc height.

Small gradient in sigma_W of 3.8 km/s per kpc.

No evidence of contamination affecting the low sigma_W values.

Abstract

We present the results of radial velocity measurements of 770 thick disk red giants toward the South Galactic Pole, vertically distributed from 0.5 kpc to 5 kpc with respect to the Galactic plane. We find a small gradient in the vertical velocity dispersion (sigma_W) of 3.8+/-0.8 km/s kpc. Even more noteworthy, our values of $\sigma_W$ are small compared to literature values: in the middle of the vertical height range we find sigma_W(z=2kpc)=30 km/s. We found no possible explanation for this small value of sigma_W in terms of sample contamination by thin disk stars, nor by wrong assumptions regarding the metallicity distribution and the derived distances.