Publications

Further observational evidence for a critical ionizing luminosity in active galaxies

Curran, S. J.; Hunstead, R. W.; Johnston, H. M.; Whiting, M. T. ; Sadler, E. M.; Allison, J. R.; Bignell, C.

We report the results of a survey for HI 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_HI ~ 10^56 ionizing photons per second (a monochromatic λ = 912 Å luminosity of L_UV ~ 10^23 W/Hz) 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.

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Matthew Whiting
Address:
Australia Telescope National Facility
PO Box 76, Epping,
NSW 1710,
Australia
Email:
Matthew.Whiting <at> csiro.au
ph: +61-2-9372-4683
fax: +61-2-9372-4444
Location:
You can now find me in room 99 at Epping.

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