Green Pea Galaxies Reveal Secrets of Ly$\hat{I}\pm$ Escape

Abstract

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

9 figures, ApJ accepted