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