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 250 ks Extended CDF-S (ECDF-S) data, and compute a 1σ upper limit on SFR X 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.