Kinematic Distances to Molecular Clouds identified in the Galactic Ring Survey

TL;DR

This study derives kinematic distances to 750 molecular clouds in the Galactic Ring Survey, resolving ambiguities using HI self-absorption and continuum absorption, and finds their distribution supports a four-arm model of the Milky Way.

Contribution

It provides a comprehensive catalog of molecular cloud distances with ambiguity resolution, linking cloud locations to Galactic spiral arms and star formation regions.

Findings

Galactic distribution of clouds aligns with four-arm spiral model.

Cloud locations match star count data from GLIMPSE.

Molecular clouds are short-lived, forming in spiral arms.

Abstract

Kinematic distances to 750 molecular clouds identified in the 13CO J=1-0 Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (BU-FCRAO GRS) are derived assuming the Clemens rotation curve of the Galaxy. The kinematic distance ambiguity is resolved by examining the presence of HI self-absorption toward the 13CO emission peak of each cloud using the Very Large Array Galactic Plane Survey (VGPS). We also identify 21 cm continuum sources embedded in the GRS clouds in order to use absorption features in the HI 21 cm continuum to distinguish between near and far kinematic distances. The Galactic distribution of GRS clouds is consistent with a four-arm model of the Milky Way. The locations of the Scutum-Crux and Perseus arms traced by GRS clouds match star count data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) star-count data. We conclude that molecular clouds must form in spiral arms and be short-lived (lifetimes < 10 Myr) in order to explain the absence of massive, 13CO bright molecular clouds in the inter-arm space.