EVOLUTIONARY PATHS ALONG THE BPT DIAGRAM FOR LUMINOUS AND ULTRALUMINOUS INFRARED GALAXIES

The evolutionary connection between nuclear starbursts and active galactic nuclei (AGNs) in luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies (ULIRGs), which result from galaxy interactions and mergers and produce the bulk of their radiation as infrared (IR) emission, is not well understood. To this effort, we present and examine new spectrophotometric data for five U/LIRGs (10(11) < L-IR < 10(13) L-circle dot) within the IRAS 2 Jy Redshift Survey with 0.05 less than or similar to z less than or similar to 0.07. We show that our sample consists almost entirely of composite objects-thus hosting both a nuclear starburst and an AGN-using the BPT diagrams. We then show that for our sample of U/LIRGs the properties that describe their nuclear starbursts and AGNs (e.g., star formation rate, L[O iii], optical D parameter, D4000, and EW(H delta)) are independent of one another, ensuring that no biases affect correlations between these parameters and the object locations on the BPT diagrams. Finally, we derive evolutionary paths on the BPT diagram involving [Nii]/H alpha that are based on how these parameters vary between two U/LIRGs positioned at the end-points of these paths. The U/LIRGs at the end-points of a given path represent the beginning and end states of a U/LIRG evolving along that path. These paths may be able to specifically explain how all local U/LIRGs evolve along the BPT diagram, and serve as a starting point for future quantitative analysis on the evolution of U/LIRGs.