Low-redshift lyα selected galaxies from galex spectroscopy: A comparison with both UV-continuum selected galaxies and high-redshift lyα emitters

Abstract

We construct a sample of low-redshift Lyα emission-line selected sources from Galaxy Evolution Explorer (GALEX) grism spectroscopy of nine deep fields to study the role of Lyα emission in galaxy populations with cosmic time. Our final sample consists of 119 (141) sources selected in the redshift interval z = 0.195-0.44 (z = 0.65-1.25) from the FUV (NUV) channel. We classify the Lyα sources as active galactic nuclei (AGNs) if high-ionization emission lines are present in their UV spectra and as possible star-forming galaxies otherwise. We classify additional sources as AGNs using line widths for our Lyα emitter (LAE) analysis. These classifications are broadly supported by comparisons with X-ray and optical spectroscopic observations, though the optical spectroscopy identifies a small number of additional AGNs. Defining the GALEX LAE sample in the same way as high-redshift LAE samples, we show that LAEs constitute only about 5% of NUV-continuum selected galaxies at z ∼ 0.3. We also show that they are less common at z ∼ 0.3 than they are at z ∼ 3. We find that the z ∼ 0.3 optically confirmed Lyα galaxies lie below the metallicity-luminosity relation of the z ∼ 0.3 NUV-continuum selected galaxies but have similar Hα velocity widths at similar luminosities, suggesting that they also lie below the metallicity-mass relation of the NUV-continuum selected galaxies. We show that, on average, the Lyα galaxies have bluer colors, lower extinctions as measured from the Balmer line ratios, and more compact morphologies than the NUV-continuum selected galaxies. Finally, we confirm that the z ∼ 2 Lyman break galaxies have relatively low metallicities for their luminosities, and we find that they lie in the same metallicity range as the z ∼ 0.3 Lyα galaxies. © 2010. The American Astronomical Society.