No X-ray-bright type II quasars among the Lyα emitters

The Lyα emitters found at z = 4.5 and 5.7 by the Large Area Lyman Alpha (LALA) survey have high equivalent widths in the Lyα line. Such lines can be produced by narrow-lined active galactic nuclei or by stellar populations with a very high proportion of young massive stars. To check for type II (i.e., narrow-lined) quasars, we obtained a deep X-ray image of 49 Lyα sources in a single field of the ACIS instrument on the Chandra X-Ray Observatory. None of these sources was detected with a 3 σ limiting X-ray luminosity of 2.9 σ 10⁴³ ergs s^-1. For comparison, the two known high-redshift type II quasars have luminosities of 4 × 10⁴³ ergs s^-1 before extinction correction. The sources remain undetected in stacked images of the 49 Lyα sources (with 6.5 Ms effective Chandra on-axis exposure) at 3 σ limits of 4.9 × 10 ⁴². The resulting X-ray-to-Lyα ratio is about 4-24 times lower than the ratio for known type II quasars, while the average Lyα luminosity of the LALA sample is between the two type II's. The cumulative X-ray-to-Lyα ratio limit is also below that of 90% of low-redshift Seyfert galaxies.