The velocity field of the outer galaxy

Abstract

Results are presented of a study of the outer Galaxy velocity field. We compiled a catalogue of HII regions and reflection nebulae with 230⁰⩽l⩽305° and |b|⩽150. For these objects heliocentric distances and radial velocities have been determined. By combining these data with similar data from the literature, for l<230°, we construct the velocity field of the outer Galaxy in the second and third quadrants. This field includes non-circular motions and is therefore very useful for the determination of kinematic distances.

A curve is fitted to the data, and it is shown that:

• 1. The rotational velocity is slightly rising as a function of galactocentric radius R, out to the last observed point (R≅20 kpc);

• 2. The rotation curves derived from objects with 0⁰⩽l<180° and 180°⩽l<360° respectively are identical, which implies the velocity field is axi-symmetric.

Non-circular motions are revealed by subtracting the velocity field, determined by the rotation curve, from the observed velocity field. The distribution of the velocity residuals, projected on the galactic plane, is consistent with a streaming pattern as predicted by the density wave theory.