Galaxy-Wide Shocks in Late-Merger Stage Luminous Infrared Galaxies

TL;DR

This study uses integral field spectroscopy to analyze shock excitation in two merging luminous infrared galaxies, revealing the importance of distinguishing shock-induced emission from star formation effects.

Contribution

It demonstrates the effectiveness of IFU data and new diagnostic models in identifying shock excitation in complex merging galaxy systems.

Findings

Widespread shock excitation is present in the studied LIRGs.

Shock excitation significantly affects emission-line diagnostics.

The results inform understanding of merger stages and galaxy evolution.

Abstract

We present an integral field spectroscopic study of two nearby Luminous Infrared Galaxies (LIRGs) that exhibit evidence of widespread shock excitation induced by ongoing merger activity, IC 1623 and NGC 3256. We show the importance of carefully separating excitation due to shocks vs. excitation by HII regions and the usefulness of IFU data in interpreting the complex processes in LIRGs. Our analysis focuses primarily on the emission line gas which is extensive in both systems and is a result of the abundant ongoing star formation as well as widespread LINER-like excitation from shocks. We use emission-line ratio maps, line kinematics, line-ratio diagnostics and new models as methods for distinguishing and analyzing shocked gas in these systems. We discuss how our results inform the merger sequence associated with local U/LIRGs and the impact that widespread shock excitation has on the interpretation of emission-line spectra and derived quantities of both local and high-redshift galaxies.