Nuclear bar, star formation, and gas fueling in the active galaxy NGC 4303

A combination of Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC 2) and Near-Infrared Camera Multiobject Spectrograph (NICMOS) images are us ed to investigate the gas/dust and stellar structure inside the central 300 pc of the nearby active galaxy NGC 4303. The NICMOS H-band (F160W) image r eveals a bright core and a nuclear-elongated barlike structure of 250 pc in diameter. The bar is centered on the bright core, and its major axis is or iented in projection along the spin axis of the nuclear gaseous rotating di sk recently detected. The V-H (F606W-F160W) image reveals a complex gas/dus t distribution with a two-arm spiral structure of about 225 pc in radius. T he southwestern arm is traced by young star-forming knots while the northea stern arm is detected by the presence of dust lanes. These spirals do not h ave a smooth structure, but rather they are made of smaller flocculent spir als or filament-like structures. The magnitudes and colors of the star-form ing knots are typical of clusters of young stars with masses of (0.5-1) x 1 0(5) M. and ages of 5-25 million years. The overall structure of the nuclea r spirals as well as the size, number, and masses of the star-forming knots are explained in the context of a massive gaseous nuclear disk subject to self-gravitational instabilities and to the gravitational held created by t he nuclear bar. According to the model, the gaseous disk has a mass of abou t 5 x 10(7) M. inside a radius of 400 pc, the bar has a radius of 150 pc an d a pattern speed of similar to 0.5 Myr(-1), and the average mass accretion rate into the core (R < 8 pc) is similar to 0.01 M. yr(-1) for about 80 My r.