Mapping the Escape Fraction of Ionizing Photons Using Resolved Stars (UVESCAPE)

The UVESCAPE team has demonstrated a new method for measuring the escape fraction of ionizing photons using HST imaging of resolved stars in NGC 4214, a local analog of high-z starburst galaxies that are thought to be responsible for cosmic reionization. Specifically, they forward model the UV through near-IR spectral energy distributions of ~83,000 resolved stars to infer their individual ionizing flux outputs using the Bayesian Extinction And Stellar Tool (BEAST; Gordon et al. 2016). They constrain the local escape fraction by comparing the number of ionizing photons produced by stars to the number that are either absorbed by dust or consumed by ionizing the surrounding neutral hydrogen in individual star-forming regions. They find substantial spatial variation in the escape fraction (0-40%). Integrating over the entire galaxy yields a global escape fraction of 25% (+16% / -15%). This value is much higher than previous escape fractions of zero reported for this galaxy. They discuss sources of this apparent tension, and demonstrate that the viewing angle and the 3D ISM geometric effects are the cause. If one assumes that NGC 4214 has no internal dust, like many high-z galaxies, they find an escape fraction of 59% (an upper limit for NGC 4214). This is the first non-zero escape fraction measurement for UV-faint (M_FUV) = -15.9 galaxies at any redshift, and supports the idea that starburst UV-faint dwarf galaxies can provide a sufficient amount of ionizing photons to the intergalactic medium. The team has made their catalog of stellar ionizing fluxes available as a High Level Science Product.