  Groups and clusters of galaxies hosting a quasar can be found through the
detection of Lyman-alpha absorption lines beyond the quasar redshift. The
effect occurs whenever the distortion to the redshift distribution of
Lyman-alpha clouds induced by the cluster potential extends beyond the
proximity effect of the quasar. Based on 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 Lyman-alpha clouds are not affected by flows on large scales (1
Mpc and more) 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 0.25[(dN/dz)/350] at $z=2$, where $dN/dz$ is the
number of Lyman-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 Lyman-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 3. This result weakens the discrepancy
between the deduced background flux and the contribution from known populations
of quasars.
