A photometric and kinematic study of the barred spiral galaxy NGC 253. II - The velocity field

The velocity field of the SAB(s)c spiral galaxy NGC 253 is derived from H-alpha Fabry-Perot interferometry covering most of the main disk of H II emission. Several basic galaxy parameters are also derived and are in good agreement with previous determinations for a systemic velocity of 236 km/s, an inclination of 78.5 deg, and a position angle of line of nodes at 51 deg. The fit of various models to the rotation curve is discussed, and the mass within the last observation at r equals 7.9 kpc is found to be 5-7 x 10 to the 10th solar masses. A two-component mass model assuming a constant M/L within a spheroid and an exponential disk cannot reproduce the observed rotation curve, and an alternative model assuming M/L equals 2 in an exponential disk component requires a massive halo with a low central mass concentration to fit the rotation curve. A pattern of noncircular motions up to plus or minus 50 km/s is also observed, and is quantitatively similar to the predicted motion of gas flow around a bar.