VLT-VIMOS integral field spectroscopy of luminous and ultraluminous
  infrared galaxies II. Evidence for shock ionization caused by tidal forces in
  the extra-nuclear regions of interacting and merging LIRGs
  LIRGs are an important class of objects in the low-z universe bridging the
gap between normal spirals and the strongly interacting and starbursting
ULIRGs. Studies of their 2D physical properties are still lacking. We aim to
understand the nature and origin of the ionization mechanisms operating in the
extranuclear regions of LIRGs as a function of the interaction phase and L_IR
by using IFS data obtained with VIMOS. Our analysis is based on over 25300
spectra of 32 LIRGs covering all types of morphologies and the entire
10^11-10^12 L_sun range. We found strong evidence for shock ionization, with a
clear trend with the dynamical status of the system. Specifically, we
quantified the variation with interaction phase of several line ratios
indicative of the excitation degree. While the [NII]/Ha ratio does not show any
significant change, the [SII]/Ha and [OI]/Ha ratios are higher for more
advanced interaction stages. We constrained the main mechanisms causing the
ionization in the extra-nuclear regions using diagnostic diagrams. Isolated
systems are mainly consistent with ionization caused by young stars. Large
fractions of the extra-nuclear regions in interacting pairs and more advanced
mergers are consistent with ionization caused by shocks. This is supported by
the relation between the excitation degree and the velocity dispersion of the
ionized gas, which we interpret as evidence for shock ionization in interacting
galaxies and advanced mergers but not in isolated galaxies. This relation does
not show any dependence with L_IR. All this indicates that tidal forces play a
key role in the origin of the ionizing shocks in the extra-nuclear regions. We
also showed what appears to be a common [OI]/Ha-sigma relation for the
extranuclear ionized gas in interacting (U)LIRGs. This needs to be investigated
further with a larger sample of ULIRGs.
