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

¹ Osservatorio di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Centro Galileo Galilei & Telescopio Nazionale Galileo, PO Box 565, 38700 S. Cruz de La Palma, Spain
³ CAISMI-CNR, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
⁵ Osservatorio Astronomico di Roma, Via Frascati 33, 00044 Rome, Italy
⁶ Università degli studi di Firenze, dipartimento di Astronomia, Largo E. Fermi 5, 50125 Firenze, Italy

Corresponding author: E. Oliva, oliva@tng.iac.es

Received: 26 January 2001
Accepted: 7 February 2001

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 [ PII] (1.188 μm) line emission. In the central the [ FeII] /[ PII] 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 [ 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 excitation in active galaxies.