On the gas surrounding high-redshift galaxy clusters

Francis & Hewett (1993) identified two 10 Mpc-scale regions of the high-red shift universe that were seemingly very overdense in neutral hydrogen. Subs equent observations showed that at least one of these gas-rich regions enve loped a cluster of galaxies at redshift 2.38. We present improved observati ons of the three background QSOs with sightlines passing within a few Mpc o f this cluster of galaxies. All three QSOs show strong neutral hydrogen abs orption at the cluster redshift, suggesting that this cluster (and perhaps all high-redshift clusters) may be surrounded by a similar to5 Mpc-scale re gion containing similar to 10(12) M-. of neutral gas. We show that if most high-redshift clusters are surrounded by such regions, the gas must be in t he form of many small (<1 kpc), dense ( >0.03 cm(-3)) clouds, each of mass < 10(6) M-.). These clouds are themselves probably gathered into >20 kpc-si zed clumps, which may be galaxy halos or protogalaxies. If this gas exists. it will be partially photoionised by the UV background. We predict the dif fuse Ly alpha flux from this photoionisation, and place observational limit s on its intensity.