Milky Way Kinematics. II. A uniform inner Galaxy HI terminal velocity curve

TL;DR

This paper presents a detailed, uniform rotation curve of the inner Milky Way using HI data, revealing well-defined velocity structures and a surprising sinusoidal pattern in the Galaxy's rotation profile.

Contribution

It provides the first densely sampled, uniform rotation curve for the inner Galaxy combining data from two quadrants, with detailed velocity structures and a new joint fit.

Findings

Well-defined velocity structures on ~200 pc scales

Rotation curves show a sinusoidal pattern between 4.2 and 7 kpc

Consistent rotation curve fit for both Galactic quadrants

Abstract

Using atomic hydrogen (HI) data from the VLA Galactic Plane Survey we measure the HI terminal velocity as a function of longitude for the first quadrant of the Milky Way. We use these data, together with our previous work on the fourth Galactic quadrant, to produce a densely sampled, uniformly measured, rotation curve of the Northern and Southern Milky Way between $3~{\rm kpc} < R < 8~{\rm kpc}$. We determine a new joint rotation curve fit for the first and fourth quadrants, which is consistent with the fit we published in McClure-Griffiths \& Dickey (2007) and can be used for estimating kinematic distances interior to the solar circle. Structure in the rotation curves is now exquisitely well defined, showing significant velocity structure on lengths of $\sim 200$ pc, which is much greater than the spatial resolution of the rotation curve. Furthermore, the shape of the rotation curves for the first and fourth quadrants, even after subtraction of a circular rotation fit shows a surprising degree of correlation with a roughly sinusoidal pattern between $4.2 < R < 7$ kpc.