Optical spectra in the non-nuclear regions of ULIRGs: Evidence of ionization by shocks

Abstract

In this contribution we present the analysis of the ionization structure of a sample of six low-z Ultraluminous Infrared Galaxies using Integral Field Spectroscopy. We discuss the possible ionizing mechanisms responsible, focussing especially on the external regions (d ∼ 2 - 10 kpc). The observed line ratios are better explained by the presence of shocks with velocities of 150 to 500 km s-1. This is supported by the existence of a positive correlation between the ionization state and the velocity dispersion, especially when using the [O I]λ6300/Hα ratio. We discuss the origin of these shocks. We find no evidence for signatures of superwinds in the extranuclear regions. We propose as an alternative explanation the existence of tidally induced large-scale gas flows caused by the merging process itself as the mechanism causing these shocks. Finally, there is one galaxy where the line ratios in the external regions cannot be explained by shocks. © 2007 Springer-Verlag Berlin Heidelberg.