PROBING EARLY CLUSTERING WITH LY-ALPHA ABSORPTION-LINES BEYOND THE QUASAR REDSHIFT

Groups and clusters of galaxies hosting a quasar can be found through the detection of Ly alpha absorption lines beyond the quasar redshift. The effect occurs whenever the distortion to the redshift distribution of Ly alpha clouds induced by the cluster potential extends beyond the proximity effect of the quasar. Based on cold dark matter (CDM) cosmological models for the evolution of structure, we predict the probability for finding lines beyond the quasar redshift (z(abs) > z(Q)) under the assumption that the physical properties of Ly alpha clouds are not affected by flows on large scales (greater than or similar to Mpc) in the quasi-linear regime. If quasars randomly sample the underlying galaxy distribution, the expected number of lines with z(abs) > z(Q) per quasar can be as high as similar to 0.5 x [(dN/dz)/350] at z = 2, where dN/dz is the number of Ly alpha lines per unit redshift far from the quasar. The probability is enhanced if quasars typically reside in small groups of galaxies. A statistical excess of Ly alpha lines is expected near very dim quasars or around metal absorption systems. Due to clustering, the standard approach to the proximity effect overestimates the ionizing background flux at high redshifts by up to a factor of similar to 3. This result weakens the discrepancy between the deduced background flux and the contribution from known populations of quasars.