X-ray nondetection of the Lyα emitters at z∼4.5

The Lyalpha emitters found at z similar to 4.5 by the Large Area Lyman Alpha (LALA) survey have high equivalent widths in the Lya 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 Lyalpha emitters. The 172 ks deep Chandra image on the LALA Bootes 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 Lyalpha sources and imaged the brightest source among our Lyalpha emitters. None of the 101 Lyalpha sources covered by two Chandra exposures were detected individually in X-rays, with a 3 sigma limiting X-ray flux of F0.5-10.0 keV < 3.3 x 10(-16) ergs cm(-2) s(-1) for on-axis targets. The sources remain undetectable in the stacked image, implying a 3 sigma limit to the average luminosity of L2-8 keV < 2.8 x 10(42) ergs s(-1). The resulting X-ray to Lyalpha 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 Lyalpha emitters at z similar to 4.5.