Mid-infrared ISO spectroscopy of NGC 4945

We have observed the central region of the nearby starburst galaxy NGC 4945 with the mid-infrared spectrometers SWS and PHT-S aboard ISO. We do not fi nd any evidence for the existence of the powerful AGN, inferred from hard X -ray observations. The upper limits on our AGN tracers [Nev]14.32 mu m&24.3 mu m and [Ne VI] 7.65 mu m imply an A(V) >160 towards the NLR, assuming th e NLR to be of equal strength as in the Circinus galaxy. Other possibilitie s are discussed. The starburst excitation indicators [Ne III] 15.56 mu m/[N e II]12.81 mu m and L-bol/L-lyc suggest that the starburst in the central r egion is at least 5x10(6) yrs old, and that it accounts for at least half o f the nuclear bolometric luminosity. The starburst might well power the ent ire bolometric luminosity, but the available constraints are also consisten t with an up to 50% contribution of the embedded AGN. With PHT-S. at a reso lution of approximate to 90, we detect strong absorption features of water ice, and, for the first time in an external galaxy, of CO2 and CO. The same PI-ITS spectrum also reveals strong emission from the family of PAH featur es. Finally, we have observed and detected several pure rotational and re-v ibrational H-2 lines, two of which, the (0-0) S(0) & S(1) lines, allow us t o determine the excitation temperature (160 K) and warm H-2 mass (2.4 x 10( 7) M.). The low excitation temperature shows Orion-like shocks not to be re presentative for the entire emission of the central region of the galaxy an d fairly normal PDRs to be perhaps more typical.