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ISO-LWS spectroscopy of Centaurus A: extended star formation
S.J. Unger 1,
P.E. Clegg 1,
G.J. Stacey 2,
P. Cox 3,
J. Fischer 4,
M. Greenhouse 5,
S.D. Lord 6,
M.L. Luhman 4,
S. Satyapal 5,
H.A. Smith 7,
L. Spinoglio 8 and
M. Wolfire 9
Received 31 August 1999 / Accepted 18 January 2000
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 " 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 [CII] line flux is 0.4% FIR and the [OI] line is 0.2%, with the remainder arising from [OIII], [NII] and [NIII ] lines. These are typical values for starburst galaxies.
The ratio of the [NIII] / [NII] line intensities from the HII regions in the dust lane corresponds to an effective temperature, K, implying that the tip of the main sequence is headed by O8.5 stars and that the starburst is 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 0.2 - 0.3 found for Galactic HII regions.
We estimate that 5% of the observed [CII] arises in the cold neutral medium (CNM) and that 10% arises in the warm ionized medium (WIM). The main contributors to the [CII ] 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 cm-3, an incident far-UV field, G times the local Galactic field, and a gas temperature of 250 K.
Key words: galaxies: individual: Centaurus A = NGC 5128 infrared: galaxies galaxies: ISM galaxies: starburst galaxies: active
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Online publication: March 21, 2000