Lyα Profile Shape as an Escape-fraction Diagnostic at High Redshift

Abstract

While the shape of the Ly α 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 α profile shape and the escape of ionizing radiation within z ∼ 3 star-forming galaxies. We measure the velocity separation between double-peaked Ly α emission structure ( v sep ), between red-side Ly α emission peaks and systemic ( v Ly α ,red ), and between red-side emission peaks and low-ionization interstellar absorption lines ( v Ly α −LIS ). We find that the IGM-corrected ratio of ionizing to nonionizing flux density is significantly higher in KLCS objects with lower v Ly α ,red . We find no significant trend between measures of ionizing-photon escape and v Ly α −LIS . We compare our results to measurements of z ∼ 0.3 “Green Peas” from the literature and find that KLCS objects have larger v sep at fixed v Ly α ,red , larger f esc at fixed v Ly α ,red , and higher v Ly α ,red overall than z ∼ 0.3 analogs. We conclude that the Ly α profile shapes of our high-redshift sources are fundamentally different, and that measurements of profile shape such as v Ly α ,red map on to f esc in different ways. We caution against building reionization-era f esc diagnostics based purely on Ly α profiles of low-redshift dwarf galaxies. Tracing v sep , v Ly α ,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.