Integral field optical spectroscopy using the INTEGRAL system, complemented with HST imaging, has been used to characterize the merging process giving rise to the ultraluminous infrared galaxy IRAS 08572+3915, the star formation along the tidal tails of the galaxy, and the nature of the nuclear ionizing sources deep within the galaxy. The overall morphology with two well-identified nuclei, the widespread star formation, and the unperturbed two-dimensional gas kinematics indicate that IRAS 08572+3915 is a dynamically young system formed by two disks' galaxies which are in the process of merging. The galaxies have a mass ratio of about 5, the brighter being a ~0.5L* galaxy. The ionized gas distribution traces the presence of young (6 Myr) dust-enshrouded massive nuclear starbursts of 2 × 107 to 10M☉. Contrary to previous claims, and based on the two-dimensional extinction-corrected optical emission line ratios, there is no evidence for a LINER or Seyfert-like nucleus in either of the galaxies. This is unusual for a warm, ultraluminous, infrared galaxy like IRAS 08572+3915. Tidal-induced, star-forming knots, located at distances of about 7 kpc from the nuclei and along the tidal tails, are traced by the presence of bright [O III]-emitting regions. These knots, with ages of 5.5-6 Myr and masses of ~106 M☉, seem to represent a common phenomenon of the merging process; they are already detected in many other ultraluminous infrared galaxies, and tidal dwarf galaxies could be just the more massive manifestation of the same phenomenon.
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