Green Pea Galaxies Reveal Secrets of Ly$\alpha$ Escape
  We analyze archival Ly$\alpha$ spectra of 12 "Green Pea" galaxies observed
with the Hubble Space Telescope, model their Ly$\alpha$ profiles with radiative
transfer models, and explore the dependence of Ly$\alpha$ escape fraction on
various properties. Green Pea galaxies are nearby compact starburst galaxies
with [OIII]$\lambda$5007 equivalent widths of hundreds of \AA. All 12 Green Pea
galaxies in our sample show Ly$\alpha$ lines in emission, with a Ly$\alpha$
equivalent width distribution similar to high redshift Ly$\alpha$ emitters.
Combining the optical and UV spectra of Green Pea galaxies, we estimate their
Ly$\alpha$ escape fractions and find correlations between Ly$\alpha$ escape
fraction and kinematic features of Ly$\alpha$ profiles. The escape fraction of
Ly$\alpha$ in these galaxies ranges from 1.4% to 67%. We also find that the
Ly$\alpha$ escape fraction depends strongly on metallicity and moderately on
dust extinction. We compare their high-quality Ly$\alpha$ profiles with single
HI shell radiative transfer models and find that the Ly$\alpha$ escape fraction
anti-correlates with the derived HI column densities. Single shell models fit
most Ly$\alpha$ profiles well, but not the ones with highest escape fractions
of Ly$\alpha$. Our results suggest that low HI column density and low
metallicity are essential for Ly$\alpha$ escape, and make a galaxy a Ly$\alpha$
emitter.
