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

TL;DR

This study uses a semi-analytical model to analyze UV spectra of Green Pea galaxies, revealing how outflow geometry and kinematics influence Ly$ extalpha$ escape and emission features.

Contribution

It introduces a detailed analysis linking outflow geometry and kinematics to Ly$ extalpha$ escape, using a semi-analytical line transfer model on Green Pea galaxy spectra.

Findings

Uncovered that partial outflow coverage enhances Ly$ extalpha$ escape.

Found that larger wind extents correlate with higher Ly$ extalpha$ escape fraction.

Suggests collisional excitation in winds can explain observed Ly$ extalpha$ luminosity.

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.