Giant cometary H ii regions and molecular bow shocks in spiral arms of galaxies: M 83

Abstract

Abstract A number of giant cometary H ii regions (GCHs) sheathed inside molecular bow shocks (MBSs) are found along spiral arms of the barred galaxy M 83. The open cone structure is explained by a model of an expanded H ii front in a gaseous arm with a steep density gradient, and the bow shock is shown to be formed by the encounter of an H ii 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 the GCH enhances star formation along the spiral arms.