BIMA CS J=2->1 observations of NGC 253: Kinematic evidence for dense gas in a bar

We present a high-resolution (6.''6 x 3.''7) CS J = 2 --> 1 map of the nearby starburst galaxy NGC 253 made with the six-element BIMA array. Dense molecular gas traced by the CS transition is seen within a similar to 30'' x 10'' (similar to 430 x 140 pc) region of the nucleus. A large-scale velocity gradient analysis estimates that the typical H-2 volume density is of the order of 3 x 10(4) cm(-3) and CS and H-2 column densities are 7 x 10(15) cm(-2) and 2 x 10(23) cm(-2) respectively. The total mass of dense gas is about 2.2 x 10(8) M(.), accounting for half the gas mass traced by CO J = 1 --> 0 emission and 17% of the dynamical mass. The morphology and kinematics of the CS emission are inconsistent with gas on circular orbits around the dynamic center. Instead, they are well explained by gas moving along closed orbits in a bar potential with the major axis of the bar at an angle of 34 degrees from the line of sight in the galactic plane. We speculate that most massive stars form in clouds on the inner x(2) orbits, which are oriented perpendicular to the bar. This provides a viable explanation for the fact that 2 cm radio continuum knots (Turner & Ho 1985) are distributed along a line perpendicular to the bar.