X-Ray Nondetection of the Lyα Emitters at z ~ 4.5

The Lyα emitters found at z ~ 4.5 by the Large Area Lyman Alpha (LALA) survey have high equivalent widths in the Lyα line, which can be produced by either narrow-lined active galactic nuclei (AGNs) or by stellar populations with a very high proportion of young, massive stars. To investigate the AGN scenario, we obtained two deep Chandra exposures to study the X-ray nature of the Lyα emitters. The 172 ks deep Chandra image on the LALA Boötes field was presented in a previous paper, and in this Letter we present a new Chandra deep exposure (174 ks) on the LALA Cetus field, which doubled our sample of X-ray-imaged Lyα sources and imaged the brightest source among our Lyα emitters. None of the 101 Lyα sources covered by two Chandra exposures were detected individually in X-rays, with a 3 σ limiting X-ray flux of F0.5-10.0 keV < 3.3 × 10-16 ergs cm-2 s-1 for on-axis targets. The sources remain undetectable in the stacked image, implying a 3 σ limit to the average luminosity of L2-8 keV < 2.8 × 1042 ergs s-1. The resulting X-ray to Lyα ratio is greater than 21 times lower than the ratios for known high-redshift type II quasars. Together with optical spectra obtained at Keck, we conclude that no evidence of AGN activity was found among our Lyα emitters at z ~ 4.5.