GREEN PEA GALAXIES REVEAL SECRETS OF Lyα ESCAPE

Abstract

We analyze archival Ly α spectra of 12 “Green Pea” galaxies observed with the Hubble Space Telescope , model their Ly α profiles with radiative transfer models, and explore the dependence of the Ly α escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [O iii ]  λ 5007 equivalent widths (EWs) of hundreds of Å. All 12 Green Pea galaxies in our sample show Ly α lines in emission, with an Ly α EW distribution similar to high-redshift Ly α emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Ly α escape fractions and find correlations between Ly α escape fraction and kinematic features of Ly α profiles. The escape fraction of Ly α in these galaxies ranges from 1.4% to 67%. We also find that the Ly α escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Ly α profiles with single H i shell radiative transfer models and find that the Ly α escape fraction anticorrelates with the derived H i column densities. Single-shell models fit most Ly α profiles well, but not the ones with the highest escape fractions of Ly α . Our results suggest that low H i column density and low metallicity are essential for Ly α escape and make a galaxy an Ly α emitter.