Ionization, Extinction, and Spiral Structure in the Inner Disk of NGC 1068

Abstract

We present our analysis of Hubble Space Telescope/Planetary Camera narrow-bandpass and broadband imagery of the inner 3 kpc region of NGC 1068. Our analysis of F160BW and F547M broadband continuum imagery suggests that roughly 40% of the scattered active galactic nucleus (AGN) continuum emission originates from an unobscured single cloud complex largely free from dust with total number densities typical of diffuse clouds in our own Milky Way. The net emission-line fluxes are extracted from continuum-subtracted narrow-bandpass imagery for Ha+[N II], Hβ, S II λλ6717, 6731, and [O III] λ5007. Although the [O III]/(Hα+[N II]) flux ratio shows a sharp drop-off at distances beyond ~4" northeast of the nucleus, the [O III]/Hb ratio indicates no such decrease. This implies that the ionization of these species is not strongly influenced by shocks associated with the expanding radio lobes as inferred from a previous study. The sharp drop-off seen in the [O III]/(Ha+[N II]) appears to be due to high interstellar reddening immediately beyond the scattering bright clouds near the nucleus, as further evidenced by the high Ha/Hb ratio in this region. The presence of a faint inner spiral arm interior to the sharply defined star formation ring, possibly driven by an outer-inner Linblad resonance, may provide a means of fueling the central AGN, as recently suggested by Yuan and Kuo. Because IR observations do not support the presence of a true AGN torus in NGC 1068, we present a qualitative model in which the radio ejecta has carved out an ionization cone in the high concentration of dense molecular clouds surrounding the nucleus. This picture also implies that the extended conical emission region to the northeast of the nucleus lies near the galactic plane and is surrounded by lower density ambient gas than that surrounding the highly ionized gas southwest of the nucleus. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555 and by STScI grant GO-5942.01-93A.