Mapping Extremely Low Metallicity Galaxies to Redshift One

We describe the results of a narrowband search for ultrastrong emission line galaxies (USELs) with EW (H beta ) greater than or equal to 30 AA. A total of 542 candidate galaxies are found in a one-half square degree survey using two similar to 120 AA filters centered at 8150 and 9140 AA with Subaru SuprimeCam. Follow-up spectroscopy for randomly selected objects in the candidate sample with Keck II DEIMOS shows that they consist of [O III] lambda 5007-, [O II] lambda 3727-, and H alpha -selected strong emission line galaxies at intermediate redshift (z < 1) and Lya-emitting galaxies at high redshift (z >> 5). We determine the H beta luminosity functions and the star formation density of the USELs, which is 5%-10% of the value found from ultraviolet continuum objects at z = 0-1, suggesting that they correspond to a major epoch in the galaxy formation process at these redshifts. Many of the USELs show the temperature-sensitive [O III] lambda 4363 auroral lines, and about a dozen have oxygen abundances satisfying the criteria of extremely metal-poor galaxies (XMPGs). These XMPGs are the most distant known today. Our high yield rate of XMPGs suggests that the narrowband method is a powerful way to find such populations. Moreover, the lowest metallicity measured In our sample is close to the minimum metallicity found in local galaxies [12 + log(O/H) similar to 7.1], although we need deeper spectra to minimize the errors. HST ACS images of several USELs exhibit widespread morphologies from relatively compact high surface brightness objects to very diffuse low surface brightness ones. The luminosities, metallicities, and star formation rates of USELs are consistent with the strong emitters being start-up intermediate-mass galaxies that will evolve into more normal galaxies and suggest that galaxies are still forming in relatively chemically pristine sites z < 1.