Nebular Emission from Simple X-ray Populations: the Impact of Ultra-luminous X-ray Sources on IR-to-optical Emission Line Diagnostics

We present a framework for modeling the contribution from ultra-luminous X-ray sources (ULXs) to the nebular emission from star-forming regions. We develop a physical model for the intrinsic ULX spectral energy distribution (SED) assuming supercritical accretion onto a stellar mass compact object, as well as a methodology to normalize the ULX SED as a function of different burst ages and metallicities using results from binary population synthesis. These models, which we refer to as “simple X-ray populations”, are then self-consistently coupled with simple stellar populations and used as input to the photoionization code Cloudy to produce large grids of nebular line and continuum emission as a function of burst age, metallicity, and ionization parameter. The results of this modeling have applications to: (1) distinguishing between different sources of hard ionizing radiation (e.g., accreting stellar versus intermediate mass black holes) via custom emission line diagnostics; (2) understanding the sources that power high ionization emission lines in high redshift galaxies and their nearby analogs, and; (3) constraining the intrinsic SED of accreting black holes across a range of black holes masses.