Composite spectra in merging U/LIRGs caused by shocks

J. A. Rich, L. J. Kewley, M. A. Dopita

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    73 Citations (Scopus)

    Abstract

    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.

    Original languageEnglish
    Article numberL12
    JournalAstrophysical Journal Letters
    Volume781
    Issue number1
    DOIs
    Publication statusPublished - 2014

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