Composite spectra in merging U/LIRGs caused by shocks

We present a key result from our optical integral field spectroscopic survey of 27 nearby ultraluminous and luminous infrared galaxies (U/LIRGs) from the Great Observatory All-Sky LIRG Survey. Using spatially resolved multi-component emission line fitting to trace the emission line ratios and velocity dispersion of the ionized gas, we quantify for the first time the widespread shock ionization in gas-rich merging U/LIRGs. Our results show a fractional contribution to the total observed Hα flux from radiative shocks increasing from a few percent during early merger stages to upward of 60% of the observed optical emission line flux in late-stage mergers. We compare our resolved spectroscopy to nuclear spectra and find that 3/4 of the galaxies in our sample that would be classified as "composite" based on optical spectroscopy are primarily characterized by a combination of star formation and merger-driven shocks. Our results have important implications for the interpretation of "composite" rest-frame optical spectra of U/LIRGs as starburst+active galactic nucleus (AGN), as the shock emission combined with star formation can mimic "composite" optical spectra in the absence of any contribution from an AGN.