X-ray properties of the z ∼ 4.5 Lyα emitters in the Chandra Deep Field South region

We report the first X-ray detection of Lyα emitters (LAEs) at redshift z ∼ 4.5. One source (J033127.2-274247) is detected in the Extended Chandra Deep Field-South (ECDF-S) X-ray data and has been spectroscopically confirmed as a z = 4.48 quasar with L_X = 4.2 × 10⁴⁴erg s^-1. The single detection gives an Lyα quasar density of ∼ 2.7^+6.2_-2.2 × 10^-6 Mpc^-3, consistent with the X-ray luminosity function of quasars. Another 22 LAEs in the central Chandra Deep Field-South region are not detected individually, but their co-added counts yield an S/N = 2.4 (p = 99.83%) detection at soft band, with an effective exposure time of ∼36 Ms. Further analysis of the equivalent width (EW) distribution shows that all the signals come from 12 LAE candidates with EW_rest < 400 Å and 2 of them contribute about half of the signal. From follow-up spectroscopic observations, we find that one of the two is a low-redshift emission-line galaxy, and the other is a Lyman break galaxy at z = 4.4 with little or no Lyα emission. Excluding these two and combined with ECDF-S data, we derive a 3σ upper limit on the average X-ray flux of F_0.5-2.0keV < 1.6 × 10^-18 ergcm^-2s^-1, which corresponds to an average luminosity of 〈L_0.5-2keV〉 < 2.4 × 10⁴² erg s^-1 for z ∼ 4.5 LAEs. If the average X-ray emission is due to star formation, it corresponds to a star formation rate (SFR) of <180-530 M⊙ yr^-1. We use this SFRX as an upper limit of the unobscured SFR to constrain the escape fraction of Lyα photons and find a lower limit of f_{esc, Lyα} > 3%-10%. However, our upper limit on the SFRX is ∼7 times larger than the upper limit on SFRX on z ∼ 3.1 LAEs in the same field and at least 30 times higher than the SFR estimated from Lyα emission. From the average X-ray-to-Lyα line ratio, we estimate that fewer than 3.2% (6.3%) of our LAEs could be high-redshift type 1 (type 2) active galactic nuclei (AGNs) and those hidden AGNs likely show low rest-frame EWs.