ISO-LWS spectroscopy of Centaurus A: extended star formation

We present the first full FIR spectrum of Centaurus A (NGC 5128) from 43 - 196.7 mu m. The data was obtained with the ISO Long Wavelength Spectrometer (LWS). We conclude that the FIR emission in a 70 " beam centred on the nucleus is dominated by star formation rather than AGN activity. The flux in the far-infrared lines is similar to 1% of the total FIR: the [C II] line flux is similar to 0.4% FIR and the [O I] line is similar to 0.2%, with the remainder arising from [O III], [N II] and [N III] lines. These are typical values for starburst galaxies. The ratio of the [N III] / [N II] line intensities from the HII regions in the dust lane corresponds to an effective temperature, T-eff similar to 35 500 K, implying that the tip of the main sequence is headed by 08.5 stars and that the starburst is similar to 6 x 10(6) years old. This suggests that the galaxy underwent either a recent merger or a merger which triggered a series of bursts. The N/O abundance ratio is consistent with the range of similar to 0.2 - 0.3 found for Galactic HII regions. We estimate that < 5% of the observed [C II] arises in the cold neutral medium (CNM) and that similar to 10% arises in the warm ionized medium (WIM). The main contributors to the [C II] emission are the PDRs, which are located throughout the dust lane and in regions beyond where the bulk of the molecular material lies. On scales of similar to 1 kPc the average physical properties of the PDRs are modelled with a gas density, n similar to 10(3) cm(-3) an incident far-UV field, G similar to 10(2) times the local Galactic field, and a gas temperature of similar to 250 K.