In the present-day universe, the highest luminosity galaxies are those with bright infrared emission. An ongoing question is what powers the infrared luminosity: star formation or growth of supermassive black holes? High-energy X-ray observations (i.e., > 10 keV) offer one tool to address this question.
In two papers we studied the X-ray emission from galaxies in the Great Observatories All-sky LIRG Survey. In “A hard X-ray view of Luminous and Ultra-luminous Infrared Galaxies in GOALS: I – AGN obscuration along the merger sequence” (Ricci+; preprint) we focus on the amount of obscuring material between us and the growing black hole. We find that the fraction of “Compton-thick” AGN (NH > 1024 cm−2) is higher when interacting galaxies are closely separated and that the CT fraction is higher than in comparison X-ray selected galaxies.
“Comprehensive Broadband X-ray and Multiwavelength Study of Active Galactic Nuclei in Local 57 Ultra/luminous Infrared Galaxies Observed with NuSTAR and/or Swift/BAT” (Yamada+, preprint) provides a similar analysis with a slightly different underlying model for the gas obscuring the X-rays. In this paper we report a similar result for the CT fraction as a function of galaxy separation, and also find that the most luminous systems tend to be accreting more rapidly and are less-obscured.
Both papers have been accepted for publication, in Monthly Notices of the Royal Astronomical Society (Ricci+) and the Astrophysical Journal Supplement Series (Yamada+).