NICS-TNG infrared spectroscopy of NGC 1068: The first extragalactic measurement of [P II] and a new tool to constrain the origin of [Fe II] line emission in galaxies

We report 0.9-1.4 mum spectroscopic observations of NGC 1068 collected duri ng the commissioning phase of the near infrared camera spectrometer (NICS) of the Telescopio Naxionale Galileo (TNG). These yielded the first extragal actic measurement of [PII] (1.188 mum) line emission. In the central 0.75 " x 2 " the [FeII]/[PII] line-intensity ratio is close to unity, similar to that measured in the Orion bar and a factor of less than or similar to 20 s maller than in supernova remnants. This indicates that most of iron is lock ed into grains and, therefore, argues against shock excitation being the pr imary origin of [FeII] line emission in the central regions of NGC 1068. We propose the [FeII]/[PII] ratio as a simple and effective tool to study and perhaps resolve the long debated questions related to the origin of [FeII] line emission and, more generally, to constrain the role of shock excitati on in active galaxies.