Highest-resolution rotation curve of the inner Milky Way proving the galactic shock wave

Abstract

Abstract We present a rotation curve (RC) of the inner Galaxy of the first quadrant at 10° ≤ l ≤ 50° (R = 1.3–6.2 kpc) with the highest spatial (2 pc) and velocity (1.3 km s−1) resolutions. We used 12CO(J = 1–0)-line survey data observed with the Nobeyama 45 m telescope at an effective angular resolution of 20″ (originally 15″), and applied the tangent–velocity method to the longitude–velocity diagrams by employing the Gaussian deconvolution of the individual CO-line profiles. A number of RC bumps, or local variation of rotation velocity, with velocity amplitudes ∼±9 km s−1 and radial scale length ∼0.5–1 kpc are superposed on the mean rotation velocity. The prominent velocity bump and corresponding density variation around R ∼ 4 kpc in the tangential direction of the Scutum arm (4 kpc molecular arm) is naturally explained by an ordinary galactic shock wave in a spiral arm with small pitch angle, not necessarily requiring a bar-induced strong shock.