Morphology and Molecular Gas Fractions of Local Luminous Infrared Galaxies as a Function of Infrared Luminosity and Merger Stage

TL;DR

This study analyzes the morphologies and molecular gas fractions of 65 local luminous infrared galaxies, revealing how merger stages influence infrared luminosity and gas content, with major mergers significantly contributing at higher luminosities.

Contribution

It provides a detailed classification of galaxy merger stages and links these stages to molecular gas fractions and infrared luminosity, offering new insights into galaxy evolution processes.

Findings

Major mergers dominate at $L_{IR} > 10^{11.5} L_\odot$

Molecular gas fraction increases with merger stage

Galaxies reach $L_{IR} > 10^{12} L_\odot$ late in the merger process

Abstract

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$_2$).