The nature of high ionization emission line galaxies near and far: Investigation of the energetic processes from stellar populations and X-ray binaries

Spectroscopic observations of high-redshift galaxies and their nearby analogs have uncovered puzzling nebular emission line features, indicative of excitation by a source of very high-energy photons. The low metallicities and bursty star formation histories of such galaxies are conducive to the efficient formation of ultra-luminous X-ray sources (ULXs), which has led to a surge of interest in whether ULXs are an important excitation mechanism in high redshift galaxies and their analogs. We present preliminary results from Chandra on the X-ray properties of a carefully selected SDSS sample of extreme emission line galaxies, whose physical properties indicate ULXs could provide a significant contribution to the ionizing photon budget. We discuss these results in the context of predictions from state-of-the-art photoionization simulations incorporating ULXs, which predict that ionizing photon production from ULXs operates most efficiently relative to stars on timescales > 10 Myr post-starburst at low metallicities. Utilizing the multi-wavelength spectro-photometric data for these galaxies, we constrain the star formation histories to present consistency checks with the photoionization models. This analysis provides insight into the major sources of high-energy ionizing photons and the host galaxy environments where they operate, which is key for interpreting spectroscopic observations of high redshift galaxies just now becoming more accessible with JWST.