Lyα EMITTING GALAXIES AS EARLY STAGES IN GALAXY FORMATION

We present optical spectroscopy of two samples of Galaxy Evolution Explorer grism selected Ly alpha emitters (LAEs): one at z = 0.195-0.44 and the other at z = 0.65-1.25. We have also observed a comparison sample of galaxies in the same redshift intervals with the same UV magnitude distributions but with no detected Ly alpha. We use the optical spectroscopy to eliminate active galactic nuclei and to obtain the optical emission-line properties of the samples. We compare the luminosities of the LAEs in the two redshift intervals and show that there is dramatic evolution in the maximum Ly alpha luminosity over z = 0-1. Focusing on the z = 0.195-0.44 samples alone, we show that there are tightly defined relations between all of the galaxy parameters and the rest-frame equivalent width (EW) of Ha. The higher EW(Ha) sources all have lower metallicities, bluer colors, smaller sizes, and less extinction, consistent with their being in the early stages of the galaxy formation process. We find that 75% +/- 12% of the LAEs have EW(H alpha) > 100 angstrom and, conversely, that 31% +/- 13% of galaxies with EW(H alpha) > 100 angstrom are LAEs. We correct the broadband magnitudes for the emission-line contributions and use spectral synthesis fits to estimate the ages of the galaxies. We find a median age of 1.1 x 10(8) yr for the LAE sample and 1.4 x 10(9) yr for the UV-continuum sample without detected Ly alpha. The median metallicity of the LAE sample is 12 + log(O/H) = 8.24, or about 0.4 dex lower than the UV-continuum sample.