MOLECULAR GASDYNAMICS OF THE YOUNG NUCLEAR STARBURST IN THE BARRED GALAXY NGC 3504

We present CO (J = 1 --> 0) interferometry at 2.5'' resolution and Hal pha CCD observations of the circumnuclear starburst region of the barr ed spiral galaxy NGC 3504. The CO emission is centrally peaked, extend s over a region 16'' (1.6 kpc) in diameter, and is relatively azimutha lly symmetric. The CO radial distribution is well fitted by an exponen tial with a scale length of 2.3'' (220 pc). This simple distribution i s surprisingly unusual for the center of a galaxy. The velocity field is consistent with purely circular motions. Gas comprises approximatel y 40% of the dynamical mass within a radius of 100 pc (1''), if the '' standard '' CO-H-2 relationship is assumed. If isothermal and self-gr avitating, the circumnuclear gas disk has a scale height of only 5-10 pc, and a spatially averaged proton density of 10(4) cm-3 at radii les s than 300 pc. The rotation curve and the dust-lane morphology indicat e the presence of an outer inner Lindblad resonance (OILR) at a radius of approximately 5'', and an inner inner Lindblad resonance (IILR) at a radius of approximately 2''. The starburst and most of the circumnu clear gas disk seem to be located between the OILR and the IILR. The m aximum value of OMEGA - kappa/2 is nearly twice as large as the bar pa ttern speed of the large-scale bar, and the OILR and the IILR are well separated, and these may be important dynamical differences between N GC 3504 and nonstarburst barred galaxies. The rate of high-mass star f ormation per unit gas mass, as traced by the ratio of Halpha to CO emi ssion, is uniformly high over the portion of the rotation curve which is nearly solid body, and drops by a factor of approximately 4 where t he rotation curve turns over and flattens out. Since the CO radial dis tribution is not ringlike despite the fact that gas is being consumed more rapidly in the center, we believe that the starburst in NGC 3504 is in an early phase of its evolution. The Toomre Q stability paramete r is approximately constant at 0.9+/-0.2 throughout the circumnuclear molecular gas disk, so the simple gravitational instability theory is consistent with ongoing star formation. The radial variation in the cl oud growth timescale predicted from a Toomre instability is similar to the radial variation in the gas depletion timescale derived from the Halpha/CO ratio, although the timescales differ by a factor of approxi mately 10(3). Either star formation is surprisingly inefficient, or th e cloud collapse timescale is longer than the instability growth times cale. We propose that the behavior of star formation for a given value of Q is strongly influenced by the strength of tidal shear, which can help control the star formation rate via the cloud destruction rate. In the central 300 pc of NGC 3504, where the rotation curve is nearly solid body and where the starburst is most intense, a lump of gas with Q congruent-to 1 has a density much greater than that which is suscep tible to tidal shear. However, 400-600 pc from the center of NGC 3504, where the rotation curve is nearly flat, a lump of gas with Q congrue nt-to 1 has a density close to the range where tidal shear can shred i t. A combination of tidal shear and gravitational instability theory c an explain why starbursts evolve from the inside out, why evolved star bursts have rings of gas where the rotation curve turns over, and why star formation and the gas supply are regulated to maintain Q congruen t-to 1 where rotation curves are nearly flat, but may be unregulated w here rotation curves are nearly solid body.