Structure and Kinematics of CO J = 2-1 Emission in the Central Region of NGC 4258

We present 12CO J = 2-1 observations toward the central region of the type 2 Seyfert galaxy NGC 4258 with the Submillimeter Array (SMA). Our interferometric maps show two armlike elongated components along the major axis of the galaxy, with no strong nuclear concentration. The CO(2-1) morphology and kinematics are similar to previous CO(1-0) results. The velocity field of the components agrees with the general galactic rotation, except for the east elongated component, which shows a significant velocity gradient along the east-west direction. In order to account for the velocity field, we propose a kinematical model in which the warped rotating disk is also expanding. The line ratio of CO(2-1)/CO(1-0) reveals that the eastern component with the anomalous velocity gradient appears to be warmer and denser. This is consistent with the gas in this component being closer to the center, being heated by the central activities, and possibly interacting with expanding motions from the nuclear region.