2002ApJ...575..826S

We report on the molecular gas properties in the central kiloparsec of the almost face-on double-barred galaxy NGC 4303 (M61), using the ¹²CO 1-0 line emission observed with the Owens Valley Radio Observatory (OVRO) millimeter interferometer. The bulk of the molecular line emission comes from two straight gas lanes that run north-south along the leading sides of the large-scale primary bar. Velocity deviations of up to 90 km.s^–1 from the mean rotational velocity are associated with these gas lanes. Inside a radius of ∼5" (∼400 pc) the molecular gas forms a spiral pattern that, for the northern arm, can be traced to the nucleus. The high angular resolution of our OVRO data (2''∼150 pc), together with archival Hubble Space Telescope (HST) data, allows for a comparison with dynamical models of gas flow in the inner kiloparsec of single- and double-barred galaxies. We find that the observed global properties of the molecular gas are in agreement with models for the gas flow in a strong, large-scale bar, and the two-arm spiral structure seen in ¹²CO in the inner kiloparsec can already be explained by a density wave initiated by the potential of that bar. Only a weak correlation between the molecular gas distribution and the extinction seen in the HST V-H map is found in the inner 400 pc of NGC 4303: The innermost part of one arm of the nuclear ¹²CO spiral correlates with a weak dust filament in the color map, while the overall dust distribution follows a ring or single-arm spiral pattern well correlated with the UV continuum. This complicated nuclear geometry of the stellar and gaseous components allows for two scenarios: (1) A self-gravitating m=1 mode is present, forming the spiral structure seen in the UV continuum. In this case, the gas kinematics would be unaffected by the small (∼4") inner bar. (2) The UV continuum traces a complete ring that is heavily extincted north of the nucleus. Such a ring forms in hydrodynamic models of double bars, but the models cannot account for the UV emission observed on the leading side of the inner bar. Comparison with other starburst ring galaxies where the molecular gas emission and the star-forming clusters form a ring or tightly wound spiral structure suggests that the starburst ring in NGC 4303 is in an early stage of formation.