Mid-infrared ISO spectroscopy of NGC 4945

Abstract

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 find any evidence for the existence of the powerful AGN, inferred from hard X-ray observations. The upper limits on our AGN tracers [ion {Ne}{v}] 14.32 mu m&24.3 mu m and [ion {Ne}{vi}] 7.65 mu m imply an A_V>160 towards the NLR, assuming the NLR to be of equal strength as in the Circinus galaxy. Other possibilities are discussed. The starburst excitation indicators [ion {Ne}{iii}]15.56 mu m/[ion {Ne}{ii}]12.81 mu m and L_bol/L_lyc suggest that the starburst in the central region is at least 5x10⁶ yrs old, and that it accounts for at least half of the nuclear bolometric luminosity. The starburst might well power the entire bolometric luminosity, but the available constraints are also consistent with an up to 50% contribution of the embedded AGN. With PHT-S, at a resolution of ~90, we detect strong absorption features of water ice, and, for the first time in an external galaxy, of CO_2 and CO. The same PHT-S spectrum also reveals strong emission from the family of PAH features. Finally, we have observed and detected several pure rotational and ro-vibrational H_2 lines, two of which, the (0-0) S(0) & S(1) lines, allow us to determine the excitation temperature (160 K) and warm H_2 mass (2.4x10⁷M_sun). The low excitation temperature shows Orion-like shocks not to be representative for the entire emission of the central region of the galaxy and fairly normal PDRs to be perhaps more typical. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA