  We describe results of a narrow band search for ultra-strong emission line
galaxies (USELs) with EW(H beta) > 30 A. 542 candidate galaxies are found in a
half square degree survey using two ~100 Angstrom 8150 A and 9140 A filters
with Subaru/SuprimeCam. Followup spectroscopy for randomly selected objects in
the sample with KeckII/DEIMOS shows they consist of [OIII] 5007, [OII] 3727,
and H alpha selected strong-emission line galaxies at intermediate redshifts (z
< 1), and Ly alpha 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 they correspond to a major epoch in galaxy formation at these
redshifts. Many USELs show the temperature-sensitive [OIII] 4363 auroral lines
and about a dozen have oxygen abundances characteristic of eXtremely Metal Poor
Galaxies (XMPGs). These XMPGs are the most distant known today. Our high yield
rate of XMPGs suggests this is a powerful method for finding such populations.
The lowest metallicity measured in our sample is 12+log(O/H) = 7.06
(6.78-7.44), close to the minimum metallicity found in local galaxies. The
luminosities, metallicities and star formation rates of USELs are consistent
with the strong emitters being start-up intermediate mass galaxies and suggest
that galaxies are still forming in relatively chemically pristine sites at z <
1.
