Molecular bow shock in the 3 kpc Norma Arm

A molecular bow shock (MBS) at G24.4+00+112 (l ∼ 24.4°, b ∼ 0°, v_LSR ∼ 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 (1.3°) 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.