Noncircular Gas Kinematics and Star Formation in the Ringed Galaxy NGC 4736

Abstract

We analyze the gas kinematics and star formation properties of the nearby RSab galaxy NGC 4736 using interferometric and single-dish CO(1-0) data and previously published Hα and H I data. The CO morphology is dominated by a central molecular bar and tightly wound spiral arms associated with a bright ring of star formation. Strong H I emission is also found in the ring, but H I is absent from the central regions. Comparison of the H I and Hα distributions suggests that H I in the ring is primarily dissociated H2. Modeling of the CO kinematics reveals gas motion in elliptical orbits around the central bar, and we argue that the ring represents both the outer Lindblad resonance of the bar and the inner Lindblad resonance of a larger oval distortion. The H I kinematics show evidence for axisymmetric inflow toward the ring and are inconsistent with streaming in aligned elliptical orbits, but the highly supersonic (~40 km s-1) inflow velocities required, corresponding to mass inflow rates of ~2 M☉ yr-1, suggest that more sophisticated models (e.g., gas orbiting in precessed elliptical orbits) should be considered. The radial CO and Hα profiles are poorly correlated in the vicinity of the nuclear bar but show a better correlation (in rough agreement with the Schmidt law) at the ring. Even along the ring, however, the azimuthal correspondence between CO and Hα is poor, suggesting that massive stars form more efficiently at some (perhaps resonant) locations than at others. These results indicate that the star formation rate per unit gas mass exhibits strong spatial variations and is not solely a function of the available gas supply. The localization of star formation to the ring is broadly consistent with gravitational instability theory, although the instability parameter Q ~ 3 on average in the ring, only falling below 1 in localized regions. Large-scale dynamical effects, by concentrating gas at resonances and influencing the star formation rate, appear to play a key role in this galaxy's evolution.