Morphology and Molecular Gas Fractions of Local Luminous Infrared
  Galaxies as a Function of Infrared Luminosity and Merger Stage
  We present a new, detailed analysis of the morphologies and molecular gas
fractions for a complete sample of 65 local luminous infrared galaxies (LIRGs)
from the Great Observatories All-Sky LIRG Survey (GOALS) using high resolution
$I$-band images from The Hubble Space Telescope, the University of Hawaii 2.2m
Telescope and the Pan-STARRS1 Survey. Our classification scheme includes single
undisturbed galaxies, minor mergers, and major mergers, with the latter divided
into five distinct stages from pre-first pericenter passage to final nuclear
coalescence. We find that major mergers of molecular gas-rich spirals clearly
play a major role for all sources with $L_{\rm IR} > 10^{11.5} L_\odot $;
however, below this luminosity threshold, minor mergers and secular processes
dominate. Additionally, galaxies do not reach $L_{\rm IR} > 10^{12.0} L_\odot $
until late in the merger process when both disks are near final coalescence.
The mean molecular gas fraction (MGF $= M_{\rm H_2} / (M_* + M_{\rm H_2})$) for
non-interacting and early-stage major merger LIRGs is 18$\pm 2$%, which
increases to 33$\pm 3$%, for intermediate stage major merger LIRGs, consistent
with the hypothesis that, during the early-mid stages of major mergers, most of
the initial large reservoir of atomic gas (HI) at large galactocentric radii is
swept inward where it is converted into molecular gas (H$_2$).
