Constraining the Star Formation Rate and AGN Fraction in IR-Luminous Merging Galaxies using SED Fitting

Ultra Luminous Infrared Galaxies (ULIRGs), galaxies with infrared (IR) luminosities ≥ 10¹² L_⊙, are thought to arise from galaxy mergers in the local universe. The extreme IR luminosities originate from both star formation (SF) and merger-triggered accretion onto supermassive black holes, when large amounts of dust absorb and re-emit ultraviolet and optical photons from these phenomena. However, it is difficult to determine which phenomenon contributes most to the IR luminosity in ULIRGs. In this study, we seek to disentangle the light emitted from SF and active galactic nuclei (AGNs) in order to estimate the contributions from each, and to better understand the star formation rates (SFRs) in merging ULIRGs, and also to understand the effect of merger stage on them. We use the Code Investigating GALaxy Emission (CIGALE) to fit the Spectral Energy Distributions (SEDs) of the galaxies used in this study. We focus mainly on constraining the far-infrared peak and turnover (FIRPT), as this feature is arguably the best indicator of the specific source of the IR luminosity. We show that supplementing Herschel PACS Photometric measurements with PACS Spectrophotometry sufficiently constrains estimates of the SF and AGN contributions to the FIRPT in ULIRGS to within 10%. We combine the results of our SED analysis for 49 galaxies with a previous sample of 189 merging ULIRGs having little to no PACS Phot data. We find that the PACS Spec data alone can be used to determine the SF and AGN contributions to the FIRPT again to within 10%. Additionally, we find that merger stage does not significantly impact the physical characteristics derived from the best fit SED models of the galaxy fluxes. This work will aid future studies in quantifying the various physical processes occurring within merging ULIRGs and help separate the different contributions from their high SFRs and/or AGNs. The SAO REU program is funded in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. AST-1852268, and by the Smithsonian Institution.