Search for tidal dwarf galaxy candidates in a sample of ultraluminous infrared galaxies

Context. Small star-forming galaxies made out of collisional debris have been found in a variety of merging systems. So far only a few of them are known in Ultraluminous Infrared Galaxies (ULIRGs) although they show clear signs of interactions. Whether external star formation may take place in such objects in an open question. Aims. The aim of this paper is to identify and characterise the physical and kinematic properties of the external star-forming regions in a sample of ULIRGs, including Tidal Dwarf Galaxies (TDG) candidates. The likelihood of survival of these regions as TDGs is also evaluated. Methods. The analysis is based on optical Integral Field Spectroscopy (IFS) and high angular resolution Hubble Space Telescope (HST) imaging. Results. We found that the presence of external star-forming regions is common, with 12 objects being identified in 5 ULIRGs. These regions show a large range of dynamical mass up to 1 x 10(10) M-circle dot, with average sizes of similar to 750 pc. In addition, the line ratios (HII region-like), metallicities (12 + log (O/H) similar to 8.6) and Ha equivalent widths (34 -257 angstrom) are typical of young bursts of star formation (age similar to 5 -8 Myr), and similar to those of the TDG candidates found in less luminous mergers and compact groups of galaxies. The extinction corrected Ha luminosity of these young bursts leads to masses for the young stellar component of similar to 2 x 10(6)-7 x 10(8) M-circle dot. The likelihood of survival of these regions as TDGs is discussed based on their structural and kinematic properties. Particularly interesting is our finding that most of these systems follow the relation between effective radius and velocity dispersion found at lower (globular clusters) and higher (Elliptical) mass systems, which suggests they are stable against internal motions. The stability against forces from the parent galaxy is studied on the basis of several criteria and a comparison of the data with the predictions of dynamical evolutionary models is also performed. Five regions out of twelve show High-Medium or High likelihood of survival based on all the utilised tracers. Our best candidate, which satisfies all criteria, is located in the advanced merger IRAS 15250+ 3609 and presents a velocity field decoupled from the relatively distant parent galaxy.