Lyα profile shape as an escape-fraction diagnostic at high redshift

Abstract

While the shape of the Ly$\alpha$ profile is viewed as one of the best tracers of ionizing-photon escape fraction ($f_{esc}$) within low redshift (z~0.3) surveys of the Lyman continuum, this connection remains untested at high redshift. Here, we combine deep, rest-UV Keck/LRIS spectra of 80 objects from the Keck Lyman Continuum Spectroscopic Survey with rest-optical Keck/MOSFIRE spectroscopy in order to examine potential correlations between Ly$\alpha$ profile shape and the escape of ionizing radiation within z~3 star-forming galaxies. We measure the velocity separation between double-peaked Ly$\alpha$ emission structure (v${}_{\mathrm{sep}}$), between red-side Ly$\alpha$ emission peaks and systemic (v${}_{\mathrm{Ly}\alpha ,\mathrm{red}}$), and between red-side emission peaks and low-ionization interstellar absorption lines (v${}_{\mathrm{Ly}\alpha -\mathrm{LIS}}$). We find that the IGM-corrected ratio of ionizing to non-ionizing flux density is significantly higher in KLCS objects with lower v${}_{\mathrm{Ly}\alpha ,\mathrm{red}}$. We find no significant trend between measures of ionizing-photon escape and v${}_{\mathrm{Ly}\alpha -\mathrm{LIS}}$. We compare our results to measurements of z~0.3 "Green Peas" from the literature and find that KLCS objects have larger v${}_{\mathrm{sep}}$ at fixed v${}_{\mathrm{Ly}\alpha ,\mathrm{red}}$, larger $f_{esc}$ at fixed v${}_{\mathrm{Ly}\alpha ,\mathrm{red}}$, and higher v${}_{\mathrm{Ly}\alpha ,\mathrm{red}}$ overall than z~0.3 analogs. We conclude that the Ly$\alpha$ profile shapes of our high-redshift sources are fundamentally different, and that measurements of profile shape such as v${}_{\mathrm{Ly}\alpha ,\mathrm{red}}$ map on to $f_{esc}$ in different ways. We caution against building reionization-era $f_{esc}$ diagnostics based purely on Ly$\alpha$ profiles of low-redshift dwarf galaxies. Tracing v${}_{\mathrm{sep}}$, v${}_{\mathrm{Ly}\alpha ,\mathrm{red}}$, and $f_{esc}$ in a larger sample of z~3 galaxies will reveal how these variables may be connected for galaxies at the epoch of reionization.

22 pages, 7 figures, submitted to ApJ