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Astron. Astrophys. 357, 24-36 (2000)


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The elusive active nucleus of NGC 4945 *

A. Marconi 1, E. Oliva 1, P.P. van der Werf 2, R. Maiolino 1, E.J. Schreier 3, F. Macchetto 3,4 and A.F.M. Moorwood 5

1 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
2 Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands
3 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
4 Affiliated to ESA science division
5 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received 27 July 1999 / Accepted 4 February 2000

Abstract

We present new HST NICMOS observations of NGC 4945, a starburst galaxy hosting a highly obscured active nucleus that is one of the brightest extragalactic sources at 100 keV. The HST data are complemented with ground based [Fe II ] line and mid-IR observations.

A 100pc-scale starburst ring is detected in [FORMULA], while [FORMULA] traces the walls of a super bubble opened by supernova-driven winds. The conically shaped cavity is particularly prominent in [FORMULA] equivalent width and in the [FORMULA] ratio. Continuum images are heavily affected by dust extinction and the nucleus of the galaxy is located in a highly reddened region with an elongated, disk-like morphology. No manifestation of the active nucleus is found, neither a strong point source nor dilution in CO stellar features, which are expected tracers of AGN activity.

Even if no AGN traces are detected in the near-IR, with the currently available data it is still not possible to establish whether the bolometric luminosity of the object is powered by the AGN or by the starburst: we demonstrate that the two scenarios constitute equally viable alternatives. However, the absence of any signature other than in the hard X-rays implies that, in both scenarios, the AGN is non-standard: if it dominates, it must be obscured in all directions, conversely, if the starburst dominates, the AGN must lack UV photons with respect to X-rays.

An important conclusion is that powerful AGNs can be hidden even at mid-infrared wavelengths and, therefore, the nature of luminous dusty galaxies cannot be always characterized by long-wavelength data alone but must be complemented with sensitive hard X-ray observations.

Key words: galaxies: active – galaxies: individual: NGC 4945 – galaxies: nuclei – galaxies: Seyfert – galaxies: starburst – infrared: galaxies

* Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. Also based on observation collected at European Southern Observatory, La Silla, Chile.

Send offprint requests to: A. Marconi

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000

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