Low-Redshift Ly-alpha Selected Galaxies from GALEX Spectroscopy: A Comparison with Both UV-Continuum Selected Galaxies and High-Redshift Ly-alpha Emitters

TL;DR

This study analyzes low-redshift Ly-alpha emitting galaxies from GALEX data, comparing their properties with UV-selected and high-redshift Ly-alpha emitters to understand their evolution and characteristics.

Contribution

It provides the first comprehensive comparison of low-redshift Ly-alpha emitters with other galaxy populations, highlighting their distinct properties and evolutionary trends.

Findings

Ly-alpha emitters are about 5% of NUV-selected galaxies at z~0.3

They are less common at z~0.3 than at z~3

Ly-alpha galaxies have lower metallicities and more compact morphologies

Abstract

We construct a sample of low-redshift Ly-alpha emission-line selected sources from GALEX grism spectroscopy of nine deep fields to study the role of Ly-alpha 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 Ly-alpha sources as 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 Ly-alpha emitter (LAE) analysis. These classifications are broadly supported by comparisons with X-ray and optical spectroscopic observations. 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 Ly-alpha galaxies lie below the metallicity-luminosity relation of the z~0.3 NUV-continuum selected galaxies but have similar H-alpha 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 Ly-alpha 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 (LBGs) have relatively low metallicities for their luminosities, and we find that they lie in the same metallicity range as the z~0.3 Ly-alpha galaxies.