The Effects of Biconical Outflows on Lyα Escape from Green Peas

Abstract

We analyze the spectra of 10 Green Pea galaxies, previously studied by Henry et al., using a semi-analytical line transfer 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 (∼10⁴ K) and warm (∼10⁵ 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 Lyα escape fraction, ${f}_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$ , as well as the Lyα emission peak separation, Δ_peak. We find that outflow geometries that leave a portion of the source uncovered along the line of sight create the best conditions for Lyα escape and have narrow peak separations, while geometries that block the observer's view of the source create the worst conditions for Lyα escape and have large peak separations. To isolate the effects of outflow kinematics, we restrict our testing set to galaxies with spherical outflows and find that ${f}_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$ and the Lyα 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 Lyα luminosity observed in these objects. Finally, we speculate on the relationship between outflows and the escape of ionizing radiation from the circumgalactic medium.