Mapping Extremely Low Metallicity Galaxies to Redshift One

Abstract

We describe the results of a narrowband search for ultrastrong emission line galaxies (USELs) with EW(Hβ)>=30 Å. A total of 542 candidate galaxies are found in a one-half square degree survey using two ~120 Å filters centered at 8150 and 9140 Å 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] λ5007-, [O II] λ3727-, and Hα-selected strong emission line galaxies at intermediate redshift (z<1) and Lyα-emitting galaxies at high redshift (z>>5). We determine the Hβ 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] λ4363 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)~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 at z<1.

Based in part on data obtained at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Based in part on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA and was made possible by the generous financial support of the W. M. Keck Foundation.