The kinematics of the ionized and molecular hydrogen in the starburst galaxy NGC 253

Near-infrared H-2 1-0 S(1) and Br gamma velocity curves along the major axis of NGC 253 have revealed a central velocity gradient that is 7 times steeper than that shown by the optical velocity curve. This is interpreted as an optical depth effect due to dust. Approximately 35 mag of visual extinction in the center is required to match the southwest side of the optical velocity curve. The spatial variation of the ratio of these lines to the (CO)-C-12 (J = 1 --> 0) line is compared among starburst galaxies NGC 253, M82, and NGC 4945 to investigate the excitation mechanism responsible for the H-2 1-0 S(1) line.