Molecular bow shock in the 3 kpc Norma Arm

Abstract

Abstract A molecular bow shock (MBS) at G24.4+00+112 (l ∼ 24${^{\circ}_{.}}$4, b ∼ 0°, vLSR ∼ 112 km s−1) is studied using the 12CO(J = 1–0)-line survey obtained with the Nobeyama 45 m telescope at 20″ (0.71 pc) resolution. The terminal velocity uniquely locates the object at the tangent point of the 3 kpc expanding arm (Norma arm) with the distance of 7.3 kpc. The bow ridge extends over ∼160 pc (1${^{\circ}_{.}}$3) perpendicularly to the galactic plane, and is concave to a ring of H ii-regions centered on G24.6+00 at the same distance. The edge on the down-stream (higher longitude) side of the MBS is extremely sharp, and is associated with several elephant trunks in gear-to-gear contact with the H ii regions. On the up-stream (lower longitude) side of MBS, a broad H i bow is associated at the same velocity. The coherently ordered structure of H i, CO, and H ii gases indicates H i-to-H2transition at the galactic shock followed by efficient star formation due to dual compression, where the molecular gas is shock-compressed from the up-stream side by galactic shock and from the down-stream side by H ii expansion. We propose a scenario of galactic sequential star formation along the spiral arms. We also discuss related ISM phenomena such as the hydraulic jump, bow shock, and Rayleigh–Taylor instability occurring around the MBS.