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Astron. Astrophys. 329, L45-L48 (1998)

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1. Introduction

The Seyfert 2 galaxy NGC 1068 is one of the nearest and brightest Seyfert galaxies and thus one of the closest candidates for an actively accreting supermassive black hole. Its distance is about 15 Mpc, corresponding to 73 pc/ [FORMULA]. Cores of Seyfert galaxies are classified as types 1 and 2, with type 1 exhibiting broad and narrow lines while the spectra of type 2 show only narrow lines. Unified theories of AGN propose that all AGN harbour a continuum source surrounded by a dusty molecular torus. Depending on the observer's viewing angle this torus either obscures the view on the inner source (type 2), or it does not (type 1) (Antonucci and Miller 1985). For NGC 1068 Bailey et al. (1988) find from NIR spectroscopy a rather low [FORMULA], perhaps as small as [FORMULA]. The extinction is wavelength-dependent and much less in the near infrared, allowing for a deeper look towards type 2 cores in the IR. Accordingly, the first high-resolution IR observations of NGC 1068 exhibited a spectacular compact central IR core and an underlying galaxy (Chelli et al. 1987; Blietz et al. 1994; Tacconi et al. 1994; Young et al. 1996; Weinberger, Neugebauer, Matthews 1996; Quirrenbach, Eckart, Thatte 1997; Thatte et al. 1997).

Recent investigations of the center of our Galaxy indicate that the difference between it and a Seyfert core is not of a generic nature but rather a question of the current level of activity (Mezger, Duschl, Zylka 1996 = MDZ96). For our Galactic Center it was shown by Duschl and Lesch (1994) and - in more detail - by Beckert et al. (1996 = BDM96) that the radio-IR spectrum of the central source Sgr A* can be explained as optically thin synchrotron radiation of quasi-monoenergetic relativistic electrons. The spectrum is characterized by a flux density [FORMULA] between a maximum flux in the FIR range and a low frequency turnover at [FORMULA] GHz due to synchrotron self-absorption (SSA).

Recently, Muxlow et al. (1996 = MPH96) have presented spectacular MERLIN interferometry observations of the central radio structure of NGC 1068 between 5 and 22 GHz. They have separated the core into 5 components with a minimum distance between individual components of [FORMULA] mas. All but one of the components show negative spectral indeces [FORMULA] between -0.33 and -0.88, while the remaining component shows a positive index of [FORMULA]. MPH96 identify this component as the true center of NGC 1068, in so far as resembling the nuclear source of our Galaxy, Sgr A*.

In this letter we present speckle masking bispectrum observations of the core of NGC 1068 in the K-band with a resolution of 76 mas, give the flux at 2.2 [FORMULA] m, and discuss the nuclear radio-IR spectrum of this Seyfert galaxy.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997