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 (MGFs) for a complete sample of 65 local luminous infrared galaxies from Great Observatories All-Sky Luminous Infrared Galaxies (LIRG) Survey using high resolution I-band images from The Hubble Space Telescope, the University of Hawaii 2.2 m 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_IR > 10^11.5L_⊙; however, below this luminosity threshold, minor mergers and secular processes dominate. Additionally, galaxies do not reach L_IR > 10^12.0L_⊙ until late in the merger process when both disks are near final coalescence. The mean MGF (MGF = M_H2/(M_∗+M_H2)) for non-interacting and early-stage major merger LIRGs is 18 ± 2%, which increases to 33 ± 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 (H₂).