1998A&A...334..783L

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 0.11arcsec, our images show that the central source is still unresolved and should be more compact than 50pc; 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 ≃2.5kpc, 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 0.15arcsec 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.