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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
Received 27 July 1999 / Accepted 4 February 2000
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 , while traces the walls of a super bubble opened by supernova-driven winds. The conically shaped cavity is particularly prominent in equivalent width and in the 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
Online publication: May 3, 2000