X-ray Constraints on the Lyman-Alpha Escape Fraction
Abstract
We have coadded X-ray flux of all known Lyman Alpha Emitters (LAEs) in the 4 Msec Chandra Deep Field South (CDF-S) region, to place sensitive upper limits on the average unobscured star-formation rate (SFRX) in these galaxies. A very small fraction of Lyman-Alpha galaxies in the field are individually detected in the X-rays, implying a low fraction of AGN 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 (ECDFS) data, and compute a 1-σ upper limit on SFRX < 14, 28, 28, 140, 440, 880 M yr-1 for LAEs located at z = 2.1, 3.1, 3.2, 4.5, 5.7 and 6.5, respectively. The upper limit of SFRX in LAEs can be then be compared to SFRLyα derived from Lyman-Alpha line and thus can constrain on the Lyman-Alpha escape fraction (fEscLyα). We derive a lower limit on f(Lyman-Alpha Escape) > 14% (84 % confidence level, 1-σ 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 SFRX well. By averaging all the LAEs at z> 2, the X-ray non-detection constrains f(Lyman-Alpha Escape) > 17% (84 % confidence level, 1-σ lower limit), and rejects f(Lyman-Alpha Escape) < 5.7% at the 99.87% confidence level from 2.1 < z < 6.5.