### Citation:

### Date Published:

July 1, 1995### Abstract:

Groups and clusters of galaxies hosting a quasar can be found throughthe detection of Lyα absorption lines beyond the quasar redshift.The effect occurs whenever the distortion to the redshift distributionof Lyα clouds induced by the cluster potential extends beyond theproximity effect of the quasar. Based on cold dark matter (CDM)cosmological models for the evolution of structure, we predict theprobability for finding lines beyond the quasar redshift(Ζ_{abs} > Ζ_{Q}) under the assumption thatthe physical properties of Lyman alpha clouds are not affected by flowson large scales (≳Mpc) in the quasi-linear regime. If quasarsrandomly sample the underlying galaxy distribution, the expected numberof lines with Ζ_{abs} > Ζ_{Q} per quasarcan be as high as ˜0.5 × [(dN/dΖ)/350] at Ζ = 2,where dN/dΖ is the number of Lyα lines per unit redshift farfrom the quasar. The probability is enhanced if quasars typically residein small groups of galaxies. A statistical excess of Lyα lines isexpected near very dim quasars or around metal absorption systems. Dueto clustering, the standard approach to the proximity effectoverestimates the ionizing background flux at high redshifts by up to afactor of ˜3. This result weakens the discrepancy between thededuced background flux and the contribution from known populations ofquasars.