Lyα Leaks in the Absorption Spectra of High-Redshift Quasi-stellar Objects

Abstract

Spectra of high-redshift QSOs show deep Gunn-Peterson absorptions on the blue sides of the Ly alpha emissions lines. They can be decomposed into components called Ly alpha leaks, defined to be emissive regions in complementary to otherwise zero-flux absorption gaps. Just like Ly alpha absorption forests at low redshifts, Ly alpha leaks are easy to find in observations and contain rich sets of statistical properties that can be used to study the early evolution of the intergalactic medium (IGM). Among all properties of a leak profile, we investigate its equivalent width in this paper, since it is weakly affected by instrumental resolution and noise. Using 10 Keck QSO spectra at z similar to 6, we have measured the number density distribution function n(W,z), defined to be the number of leaks per equivalent width W and per redshift z in the redshift range 5.4 - 6.0. These new observational statistics, in both the differential and cumulative forms, fit well to hydrodynamic simulations of uniform ionizing background in the Lambda CDM cosmology. In this model, Ly alpha leaks are mainly due to low-density voids. It supports the early studies that the IGM at z similar or equal to 6 would still be in a highly ionized state with a neutral hydrogen fraction similar or equal to 10(-4). Measurements of n(W, z) at z > 6 would be effective to probe the reionization of the IGM.