Distribution of Luminosity, Gas, and Stellar Populations in Local Luminous Infrared Galaxies as a Function of Merger Stage

Abstract

Luminous infrared galaxies (LIRGs) are galaxies where intense infrared emission is driven by star formation and active galactic nuclei. In the local universe it is clear that many LIRGs are major mergers of gas rich spiral galaxies. I have performed a careful visual classification of local (z < 0.08) LIRGs as either single non-interacting systems, minor mergers, or one of 5 major merger stages. I then used these classifications to compare galaxy merger stage with molecular gas mass, automated morphology parameters, annular optical B — I colors, and infrared surface brightness profiles. I have found that all sources above an infrared luminosity of LIR > 1011.5L⊙ are merging galaxies, while below this luminosity threshold, minor mergers and secular processes dominate. The mean molecular gas fraction (MGF = MH2(M* + MH2)) has an average value of 18±2% for non-interacting and early stage major merger LIRGs, which increases to 33±3% for intermediate stage major merger LIRGs. This is consistent with the hypotheses that during the early-mid stages of major mergers, atomic gas (H I) at large galactocentric radii is swept inward where it is converted into molecular gas (H2). The interactions also drive star formation throughout the galaxy as is evident by the blue B — I color for LIRGs at every merger stage. Late stage mergers show a reddening in their nuclear 2 kpc region, presumably also from increase in nuclear gas and dust as the galaxy nuclei coalesce. Using deep Spitzer 3.6 and 4.5 μm imaging, I find that these interactions form tidal tails and debris that extend out to 80 kpc from the galaxy nuclei. This large scale tidal debris builds up over the course of a major merger and forms up-bending infrared surface brightness profiles. I further investigate the utility of automated morphology parameters and present a refined surface brightness method for gini, M20, and concentration indices. With this new method the M20 parameter correlates with merger stage and, in combination with gini and concentration, provides a more reliable automated separation between interacting and non-interacting galaxies than previous schemes.