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Session 87 - Environments of QSOs and Radio Galaxies.
Oral session, Wednesday, January 17
Corte Real, Hilton
The environments of luminous radio-loud quasars (RLQs) evolve rapidly with redshift: at z=0.6 they are often as rich as Abell class 1 clusters, but at z<0.5 they are never so rich. This rapid, unexplained evolution indicates that the evolution of quasars is tied to their environments. The outstanding feature of quasar evolution is the huge decline in space density from z=2 to z=0, but almost nothing is known about quasar environments at z>0.7. Some z>1 quasars show possible evidence of a cluster environment in the form of ``associated'' CIV absorption lines, but this absorption might be caused by gas intrinsic to the quasars.
To study the origin of these associated absorption systems, to investigate
the environments of RLQs to high redshift, and to assemble and study a sample
of high-redshift galaxies, we are conducting an imaging search for galaxies and
galaxy clusters around a carefully selected sample of RLQs with 0.6 Candidate clusters are most easily recognized by a contrast in the number
density and/or color of galaxies near the quasar. Several candidate z<1
clusters and at least one at z>1 have been found, the latter marked by half
a dozen very red objects within 12" of a z=1.5 quasar. Less obvious candidates
are being identified via statistical comparison of galaxy counts and colors in
quasar and control fields. Studies of the photometric properties of candidate
cluster galaxies will complement similar studies in high redshift optically and
x-ray selected clusters. Knowledge of the evolution of RLQ environments to
z=2, and any dependence upon quasar luminosity, radio power, or radio spectral
index, will help constrain theories of quasar formation, fueling, and
evolution. Finally, differences between the environments of quasars with
and without associated absorption will help constrain the cluster hypothesis
for the origin of the absorbing gas.
