The Effect of Bi-conical Outflows on Ly$\alpha$ Escape From Green Peas

Abstract

We analyze the spectra of $10$ Green Pea galaxies, previously studied by Henry et al. (2015), using a semi-analytical line transfer (SALT) model to interpret emission and absorption features observed in UV galactic spectra. We focus our analysis on various ionization states of silicon, associated with the cool ($\sim 10^4$ K) and warm ($\sim 10^5$ K) gas. By analyzing low-ionization lines, we study the relationships between the distribution and kinematics of the outflowing H I gas and the observed Ly$\alpha$ escape fraction, $f_{esc}^{Ly\alpha }$, as well as the Ly$\alpha$ emission peak separation, ${\Delta }_{peak}$. We find that outflow geometries which leave a portion of the source uncovered along the line of sight create the best conditions for Ly$\alpha$ escape and have narrow peak separations, while geometries which block the observer's view of the source create the worst conditions for Ly$\alpha$ escape and have large peak separations. To isolate the effects of outflow kinematics, we restricted our testing set to galaxies with spherical outflows and found that $f_{esc}^{Ly\alpha }$ and the Ly$\alpha$ luminosity both increase with the extent of the galactic winds. A simple estimate suggests that the collisional excitation of neutral hydrogen by free electrons in the cool gas of the winds can account for the Ly$\alpha$ luminosity observed in these objects. Finally, we speculate on the relationship between outflows and the escape of ionizing radiation from the CGM.

Accepted for publication in The Astrophysical Journal

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