Giant Cometary HII Regions and Molecular Bow Shocks in Spiral Arms of Galaxies: M83

TL;DR

This paper investigates giant cometary HII regions within molecular bow shocks in the spiral arms of galaxy M83, proposing a model for their structure and implications for star formation.

Contribution

It introduces a model explaining the open cone structure of GCHs and their formation via interaction with supersonic gas flows in spiral arms.

Findings

GCHs are found inside molecular bow shocks in M83's spiral arms.

The open cone structure results from an expanded HII front in steep density gradients.

Star formation is enhanced by dual-side compression at the bow head.

Abstract

A number of giant cometary HII regions (GCH) sheathed inside molecular bow shocks (MBS) are found along spiral arms of the barred galaxy M83. The open cone structure is explained by a model of expanded HII front in a gaseous arm with steep density gradient, and the bow shock is shown to be formed by encounter of an HII region with the supersonic gas flow across the arm. It is suggested that dual-side compression of molecular gas at the bow head between the MBS and GCH enhances star formation along the spiral arms.