Green Pea Galaxies Reveal Secrets of Ly$\alpha$ Escape

TL;DR

This study analyzes Ly$ ext{alpha}$ spectra of nearby Green Pea galaxies to understand the factors influencing Ly$ ext{alpha}$ escape, revealing that low HI column density and metallicity are key for Ly$ ext{alpha}$ emission.

Contribution

It provides the first detailed analysis linking Ly$ ext{alpha}$ escape fractions to HI column density, metallicity, and dust in Green Pea galaxies using radiative transfer models.

Findings

Ly$ ext{alpha}$ escape fractions range from 1.4% to 67%.

Low HI column density correlates with higher Ly$ ext{alpha}$ escape.

Metallicity and dust influence Ly$ ext{alpha}$ escape efficiency.

Abstract

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.