What makes Ultra Luminous Infrared Galaxies shine? Properties and environments of ULIRGs from the AKARI Deep Field-South

Brackets Ultra brackets Luminous Infrared Galaxies (brackets U brackets LIRGs) are a rare class of galaxies whose exact mechanisms of activity and very high dust luminosity are still under debate, with the mixed role of galaxy mergers, starburst and Active Galactic Nucleus (AGN) to be clarified. For 39 brackets U brackets LIRGs discovered in the AKARI Deep Field-South (Malek et al. 2017) we analyzed their properties and found that ULIRGs are more commonly associated with edge-on (Type 2) AGNs than LIRGs which may point out to the geometric factor in whether we detect an object as a ULIRG or LIRG. Next, we attempted to evaluate galactic density in their vicinity based on the Digital Sky Survey (DSS) data and dedicated follow-up observations by the South African Astronomical Observatory (SAAO) telescopes. We concluded that ULIRGs prefer environments with high but not extreme density while LIRGs are more often found also in denser environments. It may imply that ULIRGs appear most often in the environment of the cluster outskirts, where galaxy mergers are the most probable, and it supports the hypothesis that mergers play the crucial role in their activation.