16213

The rest-UV emission line spectra of star-forming galaxies (SFGs) at z>6 serves as a primary tool to reveal the nature of ionizing sources that are responsible for the cosmic reionization. Low-metallicity star-forming dwarf galaxies at z<0.3 comma show similar strong UV emission lines (eg C IV comma He II comma O IIICORCHETE_CLOSE comma and C IIICORCHETE_CLOSE) comma and are being used to calibrate the UV diagnostics that can be applied to study galaxies in the reionization epoch. However comma the high equivalent widths (EW > 20 Angstrom) seen for UV emission lines in z>6 galaxies comma have not been observed in any of the local analogs. Such high EWs pose a challenge to the photoionization models comma requiring the lowest metallicities comma youngest stellar ages comma and highest ionization parameters as model inputs to produce the required hard ionizing spectrum comma and extreme emission lines. We propose to obtain HST COS (G140L) and STIS (G230L) spectra of a unique sample of 5 SFGs comma recently identified as having extreme CORCHETE_OPENOIIICORCHETE_CLOSE-CORCHETE_OPENOIICORCHETE_CLOSE ratios (>22) with evidence for hard ionizing radiation from young comma massive stars with ages < 3 Myrs. The photoionization models predict high nebular C IV and C IIICORCHETE_CLOSE EWs (> 15 Angstrom) comma comparable to galaxies in the reionization epoch. Our goal is to (1) compare the fluxes and EWs of the UV emission lines to the model predictions and infer the ISM conditions comma (2) calibrate the UV diagnostics that predict potential Lyman continuum escape comma and (3) obtain constraints on the ionizing sources (very massive stars comma WR stars comma or binaries) based on the hardness of the ionizing radiation. The results from our analysis of local analogs will offer crucial insights into the stellar populations and ISM conditions in extreme starbursts at high redshifts that are responsible for reionization.