The Virgo High-Resolution CO Survey: VI. Gas Dynamics and Star Formation along the Bar in NGC 4303

We present CO interferometer observations of the barred galaxy NGC 4303(M61). This galaxy has a gas concentration at the central region and offsetridges in the bar. Sharp velocity gradients are apparent across the ridges.Analyses of the CO data and the newborn stellar clusters revealed in HST imagesindicate the existence of unresolved molecular clouds with masses of10^4-6Msun. The observed shear velocity gradient across the ridges is too smallto break up giant molecular clouds. Therefore, the clouds are likely to survivepassage through the ridges. We discuss a cloud orbit model in a bar potential.The model reproduces the narrow offset ridges and sharp velocity gradientsacross the ridges in NGC 4303. We discuss cloud-cloud collisions (and closeinteractions) as a possible triggering mechanism for star formation. Thenewborn stellar clusters in NGC 4303 are located predominantly at the leadingsides of the offset ridges, where cloud orbits are densely populated andsuggest a high collisional frequency and possibly a high rate of triggered starformation. Cloud-based dynamics is less dissipative than smooth hydrodynamicmodels, possibly extending the timescales of gas dynamical evolution and gasfueling to central regions in barred galaxies.