ISO-LWS spectroscopy of Centaurus A: extended star formation

Abstract

We present the first full FIR spectrum of Centaurus A (NGC 5128) from 43 - 196.7 μm. The data was obtained with the ISO Long Wavelength Spectrometer (LWS). We conclude that the FIR emission in a 70 \arcsec beam centred on the nucleus is dominated by star formation rather than AGN activity. The flux in the far-infrared lines is ~ 1% of the total FIR: the [C II] line flux is ~ 0.4% FIR and the [O I] line is ~ 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 ~ 35 500 K, implying that the tip of the main sequence is headed by O8.5 stars and that the starburst is ~ 6 x 10⁶ 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 ~ 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 ~ 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 ~ 1 kpc the average physical properties of the PDRs are modelled with a gas density, n ~ 10³ cm^-3, an incident far-UV field, G ~ 10² times the local Galactic field, and a gas temperature of ~ 250 K.