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Astron. Astrophys. 327, 901-908 (1997)

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Subarcsec structures in the double nucleus of NGC 6240 disclosed with HST at 370, 430 and 500 nm *

P. Rafanelli 2, 7, H. Schulz 3, 4, 5, C. Barbieri 1, 2, S. Komossa 4, 6, U. Mebold 3, A. Baruffolo 2, 8 and M. Radovich 2

1 Member FOC Investigation Definition Team
2 Department of Astronomy, University of Padova, Vicolo Osservatorio 5, I-35122 Padova, Italy
3 Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
4 Astronomisches Institut der Ruhr-Universität, D-44780 Bochum, Germany
5 Institut für Astronomie und Astrophysik, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
6 Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
7 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
8 Osserratorio Astronomico di Padova, Vicoler Osservatorio 5, I-35122 Padova, Italy

Received 25 June 1996 / Accepted 20 January 1997

Abstract

We present an analysis of three medium band width (FWHM [FORMULA] Å) images of the nuclear region of NGC 6240 taken with the pre-COSTAR Faint Object Camera (FOC) on board of the Hubble Space Telescope. The first filter band comprises the [OII] [FORMULA] doublet, the second measures the blue continuum around 4400 Å and the third includes H [FORMULA] + [OIII] [FORMULA]. The images show that the apparent two galactic nuclei B (north) and A (south) identified from the ground consist of compact subtructures on the tenth-arcsec scale plus faint extended emission on the arcsec scale. Within a diameter of 0:0026 ( [FORMULA] pc), most compact features exhibit a summed line luminosity [FORMULA] in the range [FORMULA] - [FORMULA] erg s-1 which is comparable to that of giant HII regions.

Although the data are not complete enough for a direct decomposition into line and continuum flux, probable solutions yield a LINER like [OII]/[OIII]/H [FORMULA] line ratio for the northern compact subcomponent B1. In the southern nucleus A the faint subcomponents A2 and A3 can be classified as LINER like if they are more strongly reddened than B1 or as HII-region like if not.

The brightest southern compact component A1, however, exhibits line ratios that are typical of (i) a Seyfert galaxy or (ii) a high-excitation HII region. This leads to the interesting alternative that there is either (i) a hidden AGN (which is in accord with recent ASCA observations) or (ii) a hidden cluster of young massive stars. The second possibility would imply localized ongoing star formation within an environment whose visible light is dominated by radiation from older stellar populations. However, considering recent models for fast autoionizing shocks computed by Dopita & Sutherland (1995), A1 might also represent a shock-plus-precursor region.

Key words: galaxies: active – galaxies: interactions – galaxies: nuclei – galaxies: individual: NGC 6240

* Based on observations 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

Send offprint requests to: P. Rafanelli

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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