The Ly \alpha emitters found at z \sim 4.5 by the Large Area Lyman Alpha ( LALA ) survey have high equivalent widths in the Ly \alpha 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 Ly \alpha emitters .
The 172 ks deep Chandra image on the LALA Boötes field was presented in a previous paper ( Malhotra et al .
2003 ) , and in this paper we present a new Chandra deep exposure ( 174 ks ) on the LALA Cetus field , which doubled our sample of X-ray imaged Ly \alpha sources , and imaged the brightest source among our Ly \alpha emitters .
None of the 101 Ly \alpha sources covered by two Chandra exposures was detected individually in X-ray , with a 3 \sigma limiting X-ray flux of F _ { 0.5 - 10.0 keV } < 3.3 \times 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 L _ { 2 - 8 keV } < 2.8 \times 10 ^ { 42 } ergs s ^ { -1 } .
The resulting X-ray to Ly \alpha ratio is > 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 Ly \alpha emitters at z \sim 4.5 .