X-ray properties of the z ~ 4.5 Lyman-alpha Emitters in the Chandra Deep
  Field South Region
  We report the first X-ray detection of 113 Lyman-alpha emitters at redshift z
~ 4.5. Only 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 $L_X = 4.2\times 10^{44}$ erg/s. The single detection
gives a Lyman-alpha quasar density consistent with the X-ray luminosity
function of quasars. The coadded counts of 22 Lyman-alpha emitters (LAEs) in
the central Chandra Deep Field South (CDF-S) region yields a 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 signal
comes from 12 LAE candidates with EW_rest < 400 \AA, and 2 of them contribute
about half of the signal. Following-up spectroscopic observations show that the
two are a low-redshift emission line galaxy and a Lyman break galaxy at z =
4.4. Excluding these two and combined with ECDF-S data, we derive a 3-sigma
upper limit on the average luminosity of $L_{0.5-2 keV}$ $<$ 2.4 $\times
10^{42}$ ergs/s 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$_\sun$
per yr. We use this SFR_X as an upper limit of the unobscured SFR to constrain
the escape fraction of Lyman-alpha photons, and find a lower limit of f_esc >
3-10%. However, our upper limit on the SFR_X is ~7 times larger than the upper
limit on SFR_X on z ~ 3.1 LAEs in the same field, and at least 30 times higher
than the SFR estimated from Lyman-alpha emission. From the average X-ray to
Lyman-alpha line ratio, we estimate that fewer than 3.2% (6.3%) of our LAEs
could be high redshift type 1 (type 2) AGNs, and those hidden AGNs likely show
low rest frame EWs.
