Extranuclear Halpha-emitting complexes in low-z (U)LIRGs: Precursors of
  tidal dwarf galaxies?
  (Abridged)This paper characterizes the physical and kinematic properties of
external massive star-forming regions in a sample of (U)LIRGs. We use high
angular resolution ACS images from the HST B and I bands, as well as
Halpha-line emission maps obtained with IFS. We find 31 external
Halpha-emitting (young star-forming) complexes in 11 (U)LIRGs. These complexes
have in general similar sizes, luminosities, and metallicities to extragalactic
giant HII regions and TDG candidates found in less luminous mergers and compact
groups of galaxies. We assess the mass content and the likelihood of survival
as TDGs of the 22 complexes with simple structures in the HST images based on
their photometric, structural, and kinematic properties. The dynamical tracers
used (radius-sigma and luminosity-sigma diagrams) indicate that most of the
complexes might be self-gravitating entities. The resistance to forces from the
parent galaxy is studied by considering the tidal mass of the candidate and its
relative velocity with respect to the parent galaxy. After combining the
results of previous studies of TDG searches in ULIRGs a total of 9 complexes
satisfy most of the applied criteria and thus show a high-medium or high
likelihood of survival, their total mass likely being compatible with that of
dwarf galaxies. They are defined as TDG candidates. We propose that they
probably formed more often during the early phases of the interaction.
Combining all data for complexes with IFS data where a significant fraction of
the system is covered, we infer a TDG production rate of 0.3 candidates with
the highest probabilities of survival per system for the (U)LIRGs class. This
rate, though, might decrease to 0.1 after the systems in (U)LIRGs have evolved
for 10 Gyr, for long-lived TDGs, which would imply that no more than 5-10 % of
the overall dwarf population could be of tidal origin.
