0 Your proposal could not be saved. Submission aborted. Assigned ID: 863 ----- Attempting Submission 1 (Fri Mar 06 22:36:03 GMT 2020) ----- HST Phase I Proposal 863 successfully submitted. Receipt: # 863-1 ----- Attempting Submission 2 (Sat Mar 07 00:01:23 GMT 2020) ----- The observational consensus is that the universe was reionized by z~6, yet whether the galaxies which populated the universe at early times were capable of reionizing the Universe remains an open question. xi_ion, a conversion factor from the non-ionizing UV luminosity to the intrinsic ionizing photon production rate, is one of the key parameters to shape the history of cosmic reionization driven by galaxies, yet remains poorly constrained. Since xi_ion solely depends on the star formation physics, such as stellar age, metallicity, initial mass function, and binarity, it can be well investigated with local analogs of high-z galaxies, particularly at the UV-faint regime (Muv > -17 ) where it is hard to access at high z. To observationally constrain xi_ion for UV-faint galaxies, we will use archival HST broad-band imaging data of resolved stellar populations in local analogs that have low metallicities, low stellar masses, and bursty star formation, expected for UV-faint, but dominant, star-forming galaxies in the early universe. We will (1) measure the intrinsic ionizing photon production rate from spectral energy distribution (SED) modeling of individual stars in a given galaxy or star-forming region; (2) measure the intrinsic non-ionizing ultra-violet luminosity density from the derived star formation history in the same region; and (3) compute xi_ion by taking the ratio of these two measurements. Our program will provide the most direct observational constraint on xi_ion along with extensive analysis of uncertainties in xi_ion measurement. The PI, Yumi Choi, will lead the analysis. Our team consists of experts in resolved stellar populations and star formation in nearby dwarf galaxies (Drs. Choi, Dalcanton, Dolphin, Gilbert, Weisz, Williams), dusty ISM (Drs. Dalcanton, Gordon), and near-field cosmology (Dr. Weisz). We have many experience with large HST surveys of resolved stellar populations (e.g., ANGST, PHAT, ANGRRR, SMIDGE, METAL) and running BEAST (the stellar SED fitter) and MATCH (the CMD fitter) on those multi-band photometry datasets. Stellar Populations and the Interstellar Medium 0 Regular false false true false false SUPPORTED