Feeding the Monster: The Nucleus of NGC 1097 at Subarcsecond Scales in the Infrared with the Very Large Telescope

Near-infrared images of the prototype LINER / Seyfert 1 galaxy NGC 1097observed with the Very Large Telescope (VLT) using adaptive optics disclosewith unprecedented detail a complex central network of filamentary structurespiralling down to the center of the galaxy. The structure, consisting ofseveral spiral arms, some almost completing a revolution about the center, ismost prominent within the radius of about 300 pc. Some filaments can be tracedfurther out, where they seem to connect with the nuclear star-forming ring at0.7 kpc radius. Straight principal shocks running along the primary large-scalebar of NGC 1097, seen in the optical images as prominent dust lanes, curve intothis ring, but radio polarization vectors cross the nuclear ring under a ratherlarge angle. Here we attempt to explain this morphology in terms ofthree-dimensional gas flow in a barred galaxy. In our scenario, parts of theprincipal shock, which propagate in the off-plane gas, can cross the nuclearstar-forming ring, and excite waves inward from it. If the dispersion relationof the excited waves allows for their propagation, they will naturally take theshape of the observed central spiral. The nuclear region of NGC 1097 remainsunresolved at sub-arcsec scales in the near-IR, with an upper size limit of <10pc FWHM. Thus, any putative central dusty torus or gaseosus disk envisaged bythe AGN unified schemes has to be smaller than 10 pc in diameter at near-IRwavelengths. The extinction in the region between the nuclear star-forming ringand the nucleus increases very moderately, reaching A_v~1 at the immediatesurrounding of the nucleus. Thus, if the nuclear filaments are tracing colddust, they contribute to a very low extinction in the line of sight and arelikely to be distributed in a rather thin disk.Comment: 21 pages, 12 figures (reduced quality). Accepted for publication in Astronomical Journa