BIMA CS J = 2→ 1 OBSERVATIONS OF NGC 253: KINEMATIC EVIDENCE FOR DENSE GAS IN A BAR

摘要

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 ～30″ x 10″ (～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 solar mass , 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° 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.