A Search for Broad Infrared Recombination Lines in NGC 1068
Lutz, D.; Genzel, R.; Sturm, E.; Quirrenbach, A.; Netzer, H.; Tacconi, L.; Wagner, S. J.; Fosbury, R. A. E.; Sternberg, A.; Moorwood, A. F. M.; Wieprecht, E.; Lo, K. Y.; Fricke, K.; Alexander, T.; Awaki, H.
Germany, United States, Israel, Japan, Taiwan
Abstract
We report infrared spectroscopy of the prototypical Seyfert 2 galaxy NGC 1068, aiming at detection of broad components of hydrogen recombination lines that originate in the obscured broad-line region. Using the Short Wavelength Spectrometer on board the Infrared Space Observatory, we have observed for the first time the regions of Bβ 2.626 μm and Pfα 7.460 μm, and present improved data for Bα 4.052 μm. No significant broad components are detected, implying an equivalent visual extinction to the broad-line region of at least 50 mag and an obscuring column density of at least 1023 cm-2. While consistent with a highly obscured broad-line region, as required by the classical unified scenario, these limits are not yet significant enough to discriminate strongly between different torus models or to constrain properties of the gas causing the very large X-ray obscuration. We discuss the systematic limitations of infrared broad-line region searches and suggest that Bα may often be the most favorable transition for future searches. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries France, Germany, Netherlands, and United Kingdom) with the participation of ISAS and NASA.