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

Abstract

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 (A_V) 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 A_V(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 (A_V = 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.

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.