Composite Spectra in Merging U/LIRGs Caused by Shocks
  We present a key result from our optical integral field spectroscopic survey
of 27 nearby ultra luminous 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 \Ha flux from radiative shocks increasing
from a few percent during early merger stages to upwards 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+AGN, as the
shock emission combined with star formation can mimic "composite" optical
spectra in the absence of any contribution from an AGN.
