SEEKING THE ULTRAVIOLET IONIZING BACKGROUND AT Z-APPROXIMATE-TO-3 WITH THE KECK TELESCOPE

We describe the initial results of a deep long-slit emission-line sear ch for redshifted (2.7 < z < 4.1) Ly alpha. These observations are use d to constrain the fluorescent Lya emission from the population of clo uds whose absorption produces the higher column density component of t he Ly alpha forest in quasar spectra. We use the results to set an upp er limit on the ultraviolet ionizing background. Our spectroscopic dat a obtained with the Keck II Telescope at lambda/Delta lambda(FWHM) app roximate to 2000 reveal no candidate Ly alpha emission over the wavele ngth range of 4500-6200 Angstrom along a 3' slit in a 5400 s integrati on. This null result places the strongest limit to date on the ambient flux of Lyman continuum photons at z approximate to 3. Typically, we attain a 1 sigma surface brightness sensitivity to spectrally unresolv ed line emission in a 1 arcsec(2) aperture of (0.4-0.6) x 10(-18) ergs s(-1) cm(-2) arcsec(-2), and we search for extended emission over a w ide range of spatial scales. Our 3 a upper bound on the mean intensity of the ionizing background at the Lyman limit is J(v0) < 2 x 10(-21) ergs s(-1) cm(-2) Hz(-1) sr(-1) for 2.7 < z < 3.1 (where we are most s ensitive), assuming that Lyman limit systems have typical radii of 70 kpc (q(0) = 0.5, H-0 = 50 km s(-1) Mpc(-1)). This constraint is more t han an order of magnitude more stringent than any previously published direct limit. However, it is still a factor of 3 above the ultraviole t background level expected due to the integrated light of known quasa rs at z approximate to 3. This pilot study confirms the conclusion of Gould & Weinberg that integrations of several hours on a 10 m-class te lescope should be capable of measuring J(v0) at high redshift. Our res ults suggest that the integrated flux of Lyman continuum photons escap ing from star-forming galaxies at these epochs cannot exceed twice tha t from known quasars. We also show that it is unlikely that decaying r elic neutrinos, if composing the bulk of the dark matter, are responsi ble for the metagalactic radiation field.