Anatomy of the counterrotating molecular disk in the spiral NGC 3593 12CO(1-0) interferometer observations and numerical simulations

This paper presents high-resolution (4 " x3 ") interferometer observations of the inner disk of the starburst spiral NGC 3593, made in the (1-0) line of (CO)-C-12. NGC 3593 is an early-type system known to possess two counterrotating stellar disks of markedly different scale lengths and masses. The CO emission comes from a highly structured molecular gas disk of M-gas similar to 3 x 10(8) M., and total radial extent r similar to 35 ". The observed CO kinematics indicates that the molecular gas is counterrotating at all radii with respect to the most massive stellar disk (disk I). The bulk of the CO emission arises from a ringed circumnuclear disk (CND) of radius r similar to 10 " and mass M-gas similar to 1.5 x 10(8) M., which hosts a nuclear starburst. The link between the starburst and the CND is corroborated by high-resolution observations of other star formation tracers (H alpha, Pa alpha and J - K color index maps). The starburst episode is fueling the less massive counterrotating stellar disk (disk II). We find extinctions A(V) of similar to 1 mag in the CND based on optical and near-infrared recombination lines, but find > 5 mag from the CO and 100 mum fluxes. Out of the CND, molecular gas is distributed in a one-arm spiral feature which winds up tightly from the edges of the CND (r similar to 10 ") up to r similar to 35 ". The CO one-arm spiral is leading with respect to the gas flow in the southern half of the disk. There is a secondary trailing spiral are in the northern half The analysis of streaming motions linked with the passage of the CO one-arm spiral indicates that the southern feature would be a stationary m = 1 instability (pattern speed Omega (p) similar to 0). To account for the observed gas response in the disk of NGC 3593, we have run self-consistent numerical simulations, including the stellar and the gaseous components, in a physical scenario which approximates this case of study. We discuss the rapidly changing response of the disk, which evolves from a transitory regime, in which all instabilities are m = 1 waves leading with respect to the counterrotating gas, towards a stationary regime, in which m = 1 are mixed with m = 2 features, trailing with respect to the gas flow at all radii. In the light of the present simulations, NGC 3593 might be starting to change from the transitory towards the stationary regime.