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

Abstract

We report 0.9-1.4 μm spectroscopic observations of NGC 1068 collected during the commissioning phase of the near infrared camera spectrometer (NICS) of the Telescopio Nazionale Galileo (TNG). These yielded the first extragalactic measurement of [P II] (1.188 μm) line emission. In the central 0.75″ × 2″ the [Fe II]/[P II] line-intensity ratio is close to unity, similar to that measured in the Orion bar and a factor of ≲20 smaller than in supernova remnants. This indicates that most of iron is locked into grains and, therefore, argues against shock excitation being the primary origin of [Fe II] line emission in the central regions of NGC 1068. We propose the [Fe II]/[P II] ratio as a simple and effective tool to study and perhaps resolve the long debated questions related to the origin of [Fe II] line emission and, more generally, to constrain the role of shock excitation in active galaxies.