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 SFRX < 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 SFRX in LAEs can be then compared to SFRLyα 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 SFRX 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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science