2000A&A...363..869G

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 ¹²CO. 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∼3x10⁸M_☉, and total radial extent r∼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∼10" and mass M_gas∼1.5x10⁸M_☉, which hosts a nuclear starburst. The link between the starburst and the CND is corroborated by high-resolution observations of other star formation tracers (Hα, Paα 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 ∼1mag in the CND based on optical and near-infrared recombination lines, but find >5mag from the CO and 100µm 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∼10") up to r∼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 arc 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 Ω_p∼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.