Program: GN-2005B-Q-52

We propose to detect for the first time the Lyman-alpha fluorescence emission by HI clouds at redshift z~2.5-3 which are optically thick to ionizing radiation. The fluorescently-emitting Lyman-limit systems are expected to be extended with a median diameter of 5arcsec, and about 30 emitters should be detectable with a long (100hr) long-slit GMOS spectrum with at S/N>6x(J[912]/6.0E-22ergs/cm^2/s/Hz/sr). The universal surface brightness of the fluorescence emission directly translates to a measurement of the integrated ionizing UV background, to a level as low as J(912)~3.0E-22ergs/cm^2/s/Hz/sr. Such a measurement should settle the longstanding question to what extent galaxies and QSOs contribute to the ionizing background. Because of the extended nature of the emission, this proposal can take advantage poor seeing time and the flexibility of the Gemini queue schedule. The TAC awarded this project in 2003A/2004B and encouraged us to re-apply to complete this programme (60 of 120hours obtained so far).