Discovery of a New Population of Galactic HII Regions with Ionized Gas Velocity Gradients

TL;DR

This study uncovers a new population of Galactic HII regions exhibiting diverse velocity gradients, suggesting many are rotating and inherit angular momentum from their parent molecular clouds, based on radio recombination line observations.

Contribution

It provides the first large-scale analysis of velocity gradients in Galactic HII regions, favoring solid body rotation as the main kinematic behavior.

Findings

42% of nebulae show velocity gradients between 5 and 200 m/s/arcsec.

Approximately 15% have RRL width peaks toward the nebula centers.

Velocity gradient orientations appear random with respect to the Galactic Plane.

Abstract

We investigate the kinematic properties of Galactic HII regions using radio recombination line (RRL) emission detected by the Australia Telescope Compact Array (ATCA) at 4-10 GHz and the Jansky Very Large Array (VLA) at 8-10 GHz. Our HII region sample consists of 425 independent observations of 374 nebulae that are relatively well isolated from other, potentially confusing sources and have a single RRL component with a high signal-to-noise ratio. We perform Gaussian fits to the RRL emission in position-position-velocity data cubes and discover velocity gradients in 178 (42%) of the nebulae with magnitudes between 5 and 200 m/s/arcsec. About 15% of the sources also have a RRL width spatial distribution that peaks toward the center of the nebula. The velocity gradient position angles appear to be random on the sky with no favored orientation with respect to the Galactic Plane. We craft HII region simulations that include bipolar outflows or solid body rotational motions to explain the observed velocity gradients. The simulations favor solid body rotation since, unlike the bipolar outflow kinematic models, they are able to produce both the large, > 40 m/s/arcsec, velocity gradients and also the RRL width structure that we observe in some sources. The bipolar outflow model, however, cannot be ruled out as a possible explanation for the observed velocity gradients for many sources in our sample. We nevertheless suggest that most HII region complexes are rotating and may have inherited angular momentum from their parent molecular clouds.