X-ray constraints on the Lyα escape fraction

We have co-added the X-ray flux of all known Lyα emitters (LAEs) in the 4 Ms Chandra Deep Field South (CDF-S) region, achieving the tightest upper limits yet on the X-ray to Lyα ratio. We use the X-ray data to place sensitive upper limits on the average unobscured star formation rate (SFR _X) in these galaxies. A very small fraction of Lyα galaxies in the field are individually detected in the X-rays, implying a low fraction of active galactic nucleus activity. After excluding the few X-ray-detected LAEs, we stack the undetected LAEs located in the 4 Ms CDF-S data and 250ks Extended CDF-S (ECDF-S) data, and compute a 1σ upper limit on SFR_X < 1.6, 14, 28, 28, 140, 440, 880 M ^-1 for LAEs located at z ∼ 0.3 and z = 2.1, 3.1, 3.2, 4.5, 5.7, and 6.5, respectively. The upper limit of SFR_X in LAEs can be then compared to SFR_Lyα derived from Lyα line and thus can constrain on the Lyα escape fraction (f ^Esc _Lyα). The f ^Esc _Lyα from X-ray at z ∼ 0.3 is substantially larger than that from UV or Hα. Three X-ray-detected LAE galaxies at z ∼ 0.3 show f ^Esc _Lyα ∼ 3%-22%, and the average Lyα escape fraction from stacking the X-ray-undetected LAEs show f ^Esc _Lyα > 28% at 3σ significance level at the same redshift. We derive a lower limit on f ^Esc _Lyα > 14% (84% confidence level, 1σ lower limit) for LAEs at redshift z ∼ 2.1 and z ∼ 3.1-3.2. At z > 4, the current LAE samples are not of sufficient size to constrain SFR_X well. By averaging all the LAEs at z > 2, the X-ray non-detection constrains f ^Esc _Lyα > 17% (84% confidence level, 1σ lower limit), and rejects f ^Esc _Lyα < 5.7% at the 99.87% confidence level from 2.1 < z < 6.5.