A WESTERBORK SEARCH FOR HIGH REDSHIFT H(I)

We present the results of an extensive and sensitive search for neutra l hydrogen at a redshift of approximately 3.3 in an attempt to observe and study the properties of the precursors of rich clusters of galaxi es. We have used the Westerbork Synthesis Radio Telescope to observe, at 327 MHz, a total solid angle of approximately 32 sq. degrees (to ha lf power) with a total velocity depth ranging from 2000 to 4000 km/s. If the neutral fraction of the mass at z approximately 3.3 is sufficie ntly high one predicts structures with angular scales of a few arcminu tes and velocity dispersions of less than about 1000 km/s with flux de nsities of up to 10 mJy at 327 MHz. However, no significant positive d etections were made at spatial resolutions of 1'-8' and velocity resol utions of 200-2000 km/s at a level of 4-8 mJy. This excludes the exist ence of neutral H I masses of greater-than-or-similar-to 1-6 x 10(13)h -2M. (for OMEGA(0) = 1) at comoving number densities exceeding approxi mately 1000h3Gpc-3. Our upper limits are consistent with and confirm t he evidence from single lines of sight to quasars from which it follow s that the fraction of neutral gas must be extremely low out to z appr oximately 4-5. The implication is that some process (e.g., shock heati ng and/or the ionizing photon flux from primeval galaxies) has efficie ntly ionized the majority of the material in forming structures at z a pproximately 3.3. To be useful, further searches for neutral hydrogen at high redshift should be conducted at lower frequencies (i.e., highe r z) and/or considerably higher sensitivity.