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

摘要

LIRGs are an important class of objects in the low-z universe bridging thegap between normal spirals and the strongly interacting and starburstingULIRGs. Studies of their 2D physical properties are still lacking. We aim tounderstand the nature and origin of the ionization mechanisms operating in theextranuclear regions of LIRGs as a function of the interaction phase and L_IRby using IFS data obtained with VIMOS. Our analysis is based on over 25300spectra of 32 LIRGs covering all types of morphologies and the entire10^11-10^12 L_sun range. We found strong evidence for shock ionization, with aclear trend with the dynamical status of the system. Specifically, wequantified the variation with interaction phase of several line ratiosindicative of the excitation degree. While the [NII]/Ha ratio does not show anysignificant change, the [SII]/Ha and [OI]/Ha ratios are higher for moreadvanced interaction stages. We constrained the main mechanisms causing theionization in the extra-nuclear regions using diagnostic diagrams. Isolatedsystems are mainly consistent with ionization caused by young stars. Largefractions of the extra-nuclear regions in interacting pairs and more advancedmergers are consistent with ionization caused by shocks. This is supported bythe relation between the excitation degree and the velocity dispersion of theionized gas, which we interpret as evidence for shock ionization in interactinggalaxies and advanced mergers but not in isolated galaxies. This relation doesnot show any dependence with L_IR. All this indicates that tidal forces play akey role in the origin of the ionizing shocks in the extra-nuclear regions. Wealso showed what appears to be a common [OI]/Ha-sigma relation for theextranuclear ionized gas in interacting (U)LIRGs. This needs to be investigatedfurther with a larger sample of ULIRGs.