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Astron. Astrophys. 334, 783-788 (1998)


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Adaptive optics observations of ultra-luminous infrared galaxies I.
J, H, K images of Mkn 231 *

O. Lai 1, D. Rouan 1, F. Rigaut 2, R. Arsenault 2 and E. Gendron 1

1 Observatoire de Paris-Meudon, Département Spatial, CNRS, F-92195 Meudon Cedex, France (lai@hplyot.obspm.fr)
2 Canada-France-Hawaii Telescope Corp.

Received 4 September 1997 / Accepted 1 December 1997

Abstract

We present adaptive optics observations in J, H and K of the Ultra-Luminous Infrared Galaxy, Markarian 231. This galaxy is the one among ULIRGs that shows most clearly the character of a QSO with a very peaked nuclear source. Even with a FWHM PSF of 0.11 arcsec, our images show that the central source is still unresolved and should be more compact than 50 pc; in addition, the position on the [J-H]/[H-K] color-diagram of this point-like source is very close to the locus of QSOs. Both results point to an actual AGN - almost unreddened - at the center of Mkn 231. The underlying extended emission appears to be extremely red in a circum-nuclear region of [FORMULA] 2.5 kpc, that should be identified with the hot molecular gas disk recently found in H2. In addition, we detect on the deconvolved images a second source (B) at 0.15 arcsec north to the nucleus with [H-K] colors that are consistent either with the stellar nucleus of a merging galaxy, or with a giant HII complex of violent star formation. We favor the later interpretation since this source lies in the direction where the molecular hydrogen emission peaks.

Key words: adaptive optics – infrared: galaxies

* Based on observations obtained at Canada-France-Hawaii Telescope operated by the National Research Council of Canada, The Centre National de la Recherche Scientifique de France and the University of Hawaii.

Send offprint requests to: D. Rouan

© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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