Seeking the Ultraviolet Ionizing Background at z ≈ 3 with the Keck Telescope

We describe the initial results of a deep long-slit emission-line search for redshifted (2.7 < z < 4.1) Lyα. These observations are used to constrain the fluorescent Lyα emission from the population of clouds whose absorption produces the higher column density component of the Lyα forest in quasar spectra. We use the results to set an upper limit on the ultraviolet ionizing background. Our spectroscopic data obtained with the Keck II Telescope at λ/ΔλFWHM ≈ 2000 reveal no candidate Lyα emission over the wavelength range of 4500–6200 Å along a 3′ slit in a 5400 s integration. This null result places the strongest limit to date on the ambient flux of Lyman continuum photons at z ≈ 3. Typically, we attain a 1 σ surface brightness sensitivity to spectrally unresolved line emission in a 1 arcsec2 aperture of (0.4–0.6) × 10-18 ergs s-1 cm-2 arcsec-2, and we search for extended emission over a wide range of spatial scales. Our 3 σ upper bound on the mean intensity of the ionizing background at the Lyman limit is Jν0 < 2 × 10-21 ergs s-1 cm-2 Hz-1 sr-1 for 2.7 < z < 3.1 (where we are most sensitive), assuming that Lyman limit systems have typical radii of 70 kpc (q0 = 0.5, H0 = 50 km s-1 Mpc-1). This constraint is more than an order of magnitude more stringent than any previously published direct limit. However, it is still a factor of 3 above the ultraviolet background level expected due to the integrated light of known quasars at z ≈ 3. This pilot study confirms the conclusion of Gould &Weinberg that integrations of several hours on a 10 m–class telescope should be capable of measuring Jν0 at high redshift. Our results suggest that the integrated flux of Lyman continuum photons escaping from star-forming galaxies at these epochs cannot exceed twice that from known quasars. We also show that it is unlikely that decaying relic neutrinos, if composing the bulk of the dark matter, are responsible for the metagalactic radiation field.