Adaptive optics observations of luminous infrared galaxies - II. Imaging of the merging galaxy Arp 299

We present adaptive optics observations of the Luminous Infrared Galaxy, Ar p 299. This merging galaxy (NGC 3690) is the site of a very powerful starbu rst activity. Very high angular resolution observations at 2.2 mu m reveal many superclusters of newly formed stars. Those clusters feature intrinsic luminosities and surface densities well above the figures generally found i n the Local Cluster, pointing to a highly efficient stellar formation with possibly very high mass stars. Comparison with HST images in V band allows us to extract a [V-K] map, and shows that the extinction varies largely wit hin the area. Comparison of the very nucleus of Mkn 171A to a model point s pread function shows that it is resolved in K band, with a FWHM of 0.4" lea ding to the conclusion that an AGN as the primary power source is very unli kely. The small size, high extinction and high intrinsic luminosity of this nucleus prompts us to suggest an ultra-compact starburst as primary source of luminosity.