THE STARBURST-AGN CONNECTION IN ACTIVE GALAXIES THE MASSIVE NUCLEAR STAR-FORMING DISK IN NGC 4303

摘要

Two-dimensional spectroscopy of the low-redshift active galaxy NGC 4303 obtained with the Two- Dimensional Fiber ISIS System is presented. The ionization structure and velocity field of the nuclear region of this galaxy, where a compact UV-bright spiral structure connected with the UV-bright core has been detected with the Hubble pace Telescope (HST, Colina et al, 1997a), are investigated in detail. The ionized gas shows a structure similar to that observed in the high spatial resolution HST UV-continuum image. The Hb and Ha emission is dominated by the nuclear star-forming regions, while the core of the galaxy is the brightest region in the [O III] λ5007 and [N II] λ6584 emission lines. The optical emission line ratios of the nuclear star-forming regions are characteristic of young (ages 2-3 Myr) star-forming regions, while the core of the galaxy shows the emission-line ratios of a low-luminosity AGN in between an [O I]-weak LINER and a low-excitation Seyfert 2. The nature of the ionizing source located at the core of the galaxy is still unsolved. Its luminosity and optical emission line ratios are compatible with the presence of a young (~ 3.5 Myr) massive (~ 8 x 10~4 Mo) cluster of stars, but are also consistent with the existence of a power-law nonthermal ionizing source, Whatever the nature of this ionizing source, the nuclear star-forming spiral dominates the ionizing radiation, contributing about 90 of the total ion- icing flux. The velocity field of the ionized gas, as measured by the Hg emission line, is consistent with that of a massive rotating disk character