2017MNRAS.470.4600C

We report the results of a survey for H I 21-cm absorption at redshifts of z >= 2.6 in a new sample of radio sources with the Green Bank and Giant Metrewave Radio Telescopes. From a total of 25 targets, we report zero detections in the 16 for which optical depth limits could be obtained. Based upon the detection rate for z >= 0.1 associated absorption, we would expect approximately four detections. Of the 11 which have previously not been searched, there is sufficient source-frame optical/ultraviolet photometry to determine the ionizing photon rate for four. Adding these to the literature, the hypothesis that there is a critical rate of Q_ H  I_∼10⁵⁶ ionizing photons per second (a monochromatic λ = 912 Å luminosity of L_UV ∼ 10²³ W Hz^–1) is now significant at ≃7σ. This reaffirms our assertion that searching z >= 3 active galaxies for which optical redshifts are available selects sources in which the ultraviolet luminosity is sufficient to ionize all of the neutral gas in the host galaxy.