Streaming Motions and Kinematic Distances to Molecular Clouds

TL;DR

This study uses high-resolution simulations to analyze gas cloud motions in spiral arms, revealing systematic streaming motions and significant distance estimation errors that challenge spiral arm mapping.

Contribution

The paper provides new insights into the kinematics of molecular clouds in spiral arms and quantifies the impact of streaming motions on distance estimates.

Findings

Gas in spiral arms has a net radial streaming motion of about -9 km/s.

Kinematic distance estimates can be off by up to ±2 kpc due to streaming motions.

Streaming motions cause systematic offsets of approximately 1 kpc in distance measurements.

Abstract

We present high-resolution smoothed particle hydrodynamics simulations of a region of gas flowing in a spiral arm and identify dense gas clouds to investigate their kinematics with respect to a Milky Way model. We find that, on average, the gas in the arms can have a net radial streaming motion of $v_R \approx -9 \,\mathrm{km/s}$ and rotate $\approx 6 \,\mathrm{km/s}$ slower than the circular velocity. This translates to average peculiar motions towards the Galaxy centre and opposite to Galactic rotation. These results may be sensitive to the assumed spiral arm perturbation, which is $\approx 3\%$ of the disc potential in our model. We compare the actual distance and the kinematic estimate and we find that streaming motions introduce systematic offsets of $\approx 1$ kpc. We find that the distance error can be as large as $\pm 2$ kpc and the recovered cloud positions have distributions that can extend significantly into the inter-arm regions. We conclude that this poses a difficulty in tracing spiral arm structure in molecular cloud surveys.