Merging process and tidal-induced star formation in the ultraluminous infrared galaxy IRAS 08572+3915

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 format ion along the tidal tails of the galaxy, and the nature of the nuclear ioni zing sources deep within the galaxy. The overall morphology with two well-i dentified nuclei, the widespread star formation, and the unperturbed two-di mensional gas kinematics indicate that IRAS 08572+3915 is a dynamically you ng 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 similar t o0.5L* galaxy. The ionized gas distribution traces the presence of young (6 Myr) dust-enshrouded massive nuclear starbursts of 2 x 10(7) to 10(8) M.. Contrary to previous claims, and based on the two-dimensional extinction-co rrected optical emission line ratios, there is no evidence for a LINER or S eyfert-like nucleus in either of the galaxies. This is unusual for a warm, ultraluminous, infrared galaxy like IRAS 08572+3915. Tidal-induced, star-fo rming 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 regi ons. These knots, with ages of 5.5-6 Myr and masses of similar to 10(6) see m to M., represent a common phenomenon of the merging process; they are alr eady detected in many other ultraluminous infrared galaxies, and tidal dwar f galaxies could be just the more massive manifestation of the same phenome non.