High ionization emission line galaxies near and far: A reckoning of all the energetic processes

The epoch of Reionization and Cosmic Dawn (z>6) is the next big frontier for galaxy studies, brought into focus by the highly anticipated data from JWST and upcoming 21-cm studies (HERA, SKA). Understanding which sources are relevant to ionization at these great distances requires a detailed investigation of energetic processes (e.g., star formation and black hole accretion) in young, very low metallicity regions. In both the high-z galaxies and nearby He II emitters, the high-ionization line ratios challenge our understanding about the energetic sources capable of producing such emission lines. I will present our work on high-ionization line modeling including contributions from stellar and X-ray binary photoionization and shock ionization. We compare these models with observations of known He-II sources, and can further constrain the models for individual sources using multi-wavelength photometry to study their ionizing sources in more detail. As the final talk of the session, which focuses on the efforts underway to understand the energetic sources present in high-redshift galaxies and their analogs, I will summarize the current landscape and outline future avenues of research for this relevant and timely topic.