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

1. Introduction

NGC 4945 is a nearby, large (204´) spiral galaxy seen nearly edge on (i 78^o; Ott 1995). At a recession velocity of 560 km s^-1 it is at the mean radial velocity of the Centaurus group (Hesser et al. 1984), of which it is believed to be a member. Distance estimates vary between 3.5 and 4.0Mpc (see Bergman et al. 1992and Mauersberger et al. 1996for discussions). In this paper we will adopt a distance of 3.9 Mpc (Bergman et al. 1992), which implies that 1" is equivalent to 18 pc.

NGC 4945 is one of the brightest infrared galaxies in the sky: S[12]=24 Jy, S[25]=43 Jy, S[60]=588 Jy, S[100]=1416 Jy (Rice et al. 1988). The total infrared luminosity amounts to L(8-1000 µm)=2.9510¹⁰ , 75% of which originates in the central 129" (Brock et al. 1988).

Near infrared observations reveal the nuclear region to be the site of a powerful, yet visually obscured, starburst. Br (Moorwood et al. 1996a) and Pa (Marconi et al. 2000) recombination line maps show the starburst to be concentrated in a circumnuclear disk or ring 200 pc across (11"). Further evidence for (a period of) strong star formation comes from the discovery of a conical structure, roughly perpendicular to the galaxy major axis. It is believed to be a cavity, vacated by a starburst-driven superwind (Heckman et al. 1990; Moorwood et al. 1996a). The non-detection of [OIII ] within the cone and the absence of coronal lines excludes an AGN as the driver of the outflow.

Clear evidence for the presence of an AGN comes from hard X-ray observations (Iwasawa et al. 1993; Guainazzi et al. 2000). The AGN X-ray emission is however heavily absorbed by a column density of 10^24.7 cm^-2, which obscures the AGN at all optical and infrared wavelengths. Previous authors have attributed most of the IR luminosity to the starburst (e.g. Moorwood & Oliva 1994; Koornneef & Israel 1996). Hard X-ray observations with BeppoSAX indicate that the bolometric luminosity may as well be accounted for by the AGN alone (Guainazzi et al. 2000).

3 cm&6 cm ATCA radio maps of the central region of NGC 4945 (Forbes & Norris 1998) are dominated by strong nuclear emission, and emission extended along the disc of the galaxy. There is also evidence for some filamentry structure associated with the cavity cleared by the starburst superwind. VLBI observations by Sadler et al. (1995) reveal the existence of a compact radio core. This, as well as the presence of H₂O megamasers in a Keplerian disc about a 10⁶ black hole (Greenhill et al. 1997), are taken as further evidence for the presence of an AGN.

Near infrared observations of molecular hydrogen emission in NGC 4945 have been reported by several authors over the last 15 years (e.g. Moorwood & Glass 1984; Moorwood & Oliva 1988; Koornneef 1993; Moorwood & Oliva 1994; Koornneef & Israel 1996; Moorwood et al. 1996a; Quillen et al. 1999; Marconi et al. 2000). While fluxes are known for eight ro-vibrational transitions accesible from the ground (Koornneef & Israel 1996), spatial information is available only for the (1-0) S(1) 2.1218 µm line. These observations show the H₂ emission to be associated with the hollow cone, not with the starburst traced in hydrogen recombination emission. The absence of a correlation argues against photons as the source of excitation. Instead, the emission is attributed to shock heating of the molecular material at the face of the cavity (Moorwood et al. 1996a; Marconi et al. 2000).

Mid-infrared spectroscopy is much less affected by intervening extinction than the UV and optical equivalents, with A()/A_V less than 0.1. Observations of the central region of the galaxy, using the mid-infrared spectrometer SWS (De Graauw et al. 1996) and the spectrophotometer PHT-S (Lemke et al. 1996), both aboard ISO (Kessler et al. 1996), are therefore very useful to study the nuclear components otherwise hidden by heavy extinction. In Sect. 3.1 we present the results of the search for high excitation emission from the AGN. In Sect. 3.2 we study the properties of the nuclear starburst. In Sect. 3.3 we discuss the dominant nuclear power source. Sect. 3.4 discusses the broad emission and absorption features, tracing the properties of the interstellar medium in and in front of the nucleus. Finally, in Sect. 3.5 we discuss the physical conditions and excitation of the warm molecular hydrogen.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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