Non-circular 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 Halpha and HI 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 HI emission is also found in the ring, but HI is absent from the central regions. Comparison of the HI and Halpha distributions suggests that HI in the ring is primarily dissociated H$_2$. Modeling of the CO kinematics reveals gas motion in elliptical orbits around the central bar, and we argue that the ring represents both the OLR of the bar and the ILR of a larger oval distortion. The HI kinematics show evidence for axisymmetric inflow towards the ring and are inconsistent with streaming in aligned elliptical orbits, but the highly supersonic (~40 km/s) inflow velocities required, corresponding to mass inflow rates of ~2 Msol/yr, suggest that more sophisticated models (e.g., gas orbiting in precessed elliptical orbits) should be considered. The radial CO and Halpha 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 Halpha 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 $Q \sim 3$ on average in the ring, only falling below 1 in localized regions.