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

Mid-infrared ISO spectroscopy of NGC 4945 ^*

H.W.W. Spoon ¹, J. Koornneef ², A.F.M. Moorwood ¹, D. Lutz ³ and A.G.G.M. Tielens <sup>2

1</sup> European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany (hspoon,amoor@eso.org)
² Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands (j.koornneef,tielens@astro.rug.nl)
³ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1603, 85740 Garching, Germany (lutz@mpe.mpg.de)

Received 28 January 2000 / Accepted 20 March 2000

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 14.32 µm&24.3 µm and 7.65 µm imply an A160 towards the NLR, assuming the NLR to be of equal strength as in the Circinus galaxy. Other possibilities are discussed. The starburst excitation indicators 15.56 µm/12.81 µm and suggest that the starburst in the central region is at least 510⁶ 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 CO₂ 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 H₂ lines, two of which, the (0-0) S(0) & S(1) lines, allow us to determine the excitation temperature (160 K) and warm H₂ mass . 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.

Key words: galaxies: individual: NGC 4945 galaxies: ISM galaxies: starburst galaxies: Seyfert

* 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

Send offprint requests to: H.W.W. Spoon

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000

helpdesk@link.springer.de