Astron. Astrophys. 357, 898-908 (2000)

4. Conclusions

The main results of this paper can be summarized as follows:

• The nuclear starburst is heavily obscured by 36 mag. of visual extinction, as infered from the 18.7 µm/33.5 µm ratio.

• The excitation of the nuclear starburst is very low, as deduced from excitation indicators 15.56 µm/ 12.81 µm and , consistent with an age of at least 510⁶ yrs. Comparison with starburst models implies that at least 50% of the bolometric luminosity is powered by the starburst.

• The very low inferred black hole mass, the very cold mid-infrared to far-infrared colors, and the absence of any free line of sight to the NLR supports the conclusion that the starburst dominates the bolometric luminosity.

• Our mid-infrared ISO spectroscopy does not provide any evidence for the existence of an AGN in the nucleus of NGC 4945. The only high excitation line detected, the 25.9 µm line, is most likely produced in shocks associated with the nuclear starburst.

• The AGN, detected in hard X-rays, is unusual in not revealing itself at optical, near-infrared and mid-infrared wavelengths. Hence, either the NLR is extremely obscured , or UV photons from the AGN are absorbed close to the nucleus along all lines of sight, or the AGN is deficient in UV relative to its X-ray flux.

• Many ISM solid state and molecular features have been observed with ISO-PHT-S in the 2.4-11.7 µm range. Most prominent in emission are the PAH features at 3.3, 6.2, 7.7 and 11.2 µm. The strongest absorption features are those of water ice, CO₂ and CO, seen against the nuclear spectrum. These features show striking similarities to the absorption features seen towards the Galactic center.

• We have studied the physical conditions, excitation and mass of warm H₂, combining IRSPEC and ISO observations of 14 transitions. We derive a visual extinction of 20 mag. to the H₂ emitting region. From the (0-0) S(0)& S(1) lines, we compute a warm (160 K) H₂ gas mass of , 9% of the total gas mass inferred from CO. The excitation diagram is best fitted by a power law of the form dM/dT=4.4310¹⁵ T^-4.793 . The low excitation temperature of 160 K shows Orion-like shocks not to be representative for the entire emission, and fairly normal PDRs to be perhaps more typical.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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