The deeply obscured AGN of NGC 4945⋆
I. Spitzer-IRS maps of [Ne V], [Ne II], H2 0−0 S(1), S(2), and other tracers
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69,
2 Astronomy Department, Cornell University, Ithaca, NY, USA
3 Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, 9747 AD Groningen, The Netherlands
4 Department of Physics and Astronomy, Mail Drop 111, University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606, USA
Received: 28 April 2011
Accepted: 1 July 2011
Context. The nearly edge-on galaxy NGC 4945 is one of the closest galaxies where an AGN and starburst coexist, and is one of the brightest sources at 100 keV. Near and mid-infrared spectroscopy have shown very strong obscuration of its central region, rivaled only in strength by some of the most deeply obscured ULIRGs.
Aims. We determine the spatial distribution of ISM emission features in the central 426 × 426 pc2 of NGC 4945.
Methods. We mapped the central region of NGC 4945 in three of the four Spitzer-IRS modules (SH, SL and LL). In particular, we produced maps of the flux distribution of the starburst tracers [Ne II], [Ne III], [S III] and [S IV] at 12.81, 15.56, 18.71, and 10.51 μm, respectively, and a map of the AGN narrow-line region tracer [Ne V] at 14.32 μm. In addition, we mapped the S(1), S(2), and S(3) pure rotational lines of H2, which trace the distribution of warm molecular hydrogen. Finally, we obtained an extinction map (AV) based on the apparent strength of the 9.7 μm silicate absorption feature.
Results. At a spatial resolution of ~5′′, our extinction map traces the contours of the starburst ring. The highest extinction is, however, found at the nucleus, where we measure AV(9.85 μm) ≈ 60. Within the uncertainty of the astrometry, all emission lines are found to peak on the nucleus, except for the warm molecular hydrogen emission, which shows a maximum 60−100 pc NW of the nucleus. We favor a scenario in which the emission of the lower H2 0−0 S(1) and S(2) rotational lines originate mainly in an unobscured extranuclear component associated with the super-wind cone observed in the HST NICMOS map of the H2 1−0 S(1) vibrational line. For the [Ne V] emission we infer an attenuation of a factor 12−160 (AV = 55−112) based on a comparison of the ratio of our [Ne V] flux and the absorption-corrected 14–195 keV Swift-BAT flux to the average [Ne V]/BAT ratio for Seyfert 1 nuclei. The high attenuation indicates that [Ne V] and [O IV] cannot be used as extinction-free tracers of AGN power in galaxies with deeply buried nuclei.
Key words: galaxies: active / galaxies: nuclei / galaxies: individual: NGC 4945 / ISM: lines and bands
We dedicate this paper to the memory of our esteemed colleague and advisor, Alan Moorwood (1945−2011), who pioneered the infrared spectroscopic study of NGC 4945. This work is based on observations obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407.
© ESO, 2011