QUASISTELLAR OBJECTS, IN GALAXY CLUSTERS AND
SUPERCLUSTERS

PATRICK S. OSMER

Quasars often  occur in groups  of  galaxies but  rarely are found  in
clusters of galaxies  at redshifts less  than 0.5. At larger redshifts
certain  types of quasars do  occur in galaxy  clusters, and groups of
quasars the size of galaxy clusters and superclusters are being found.
Quasars are the only available tracers of the large-scale structure of
the universe  at redshifts larger than  1 because normal  galaxies are
too faint to study. The relation of quasars to neighboring galaxies is
important to the study of the formation  and evolution of both quasars
and galaxies.

BACKGROUND

Although quasars were originally defined as star-like objects of large
redshift,   application of   higher-resolution  imaging showed    that
extended  emission  occurred around quasars  with  redshifts less than
0.5.  Since then, the generally accepted  picture of quasars being the
extremely luminous nuclei of   galaxies has been developed.  With  the
realization that quasars occur in galaxies came a number of questions:
Do quasars occur in groups or clusters of galaxies? What can they tell
us  about the conditions   in such groups  and  clusters? What can  be
learned about the formation and evolution of quasars by studying their
environments? Does the  spatial distribution of quasars  provide clues
to the large-scale structure of the universe  and its evolution? There
has been considerable  progress on some  of these  questions in recent
years.   Others are just now  being studied,  and substantial progress
can be expected in the future.

  This  entry first gives  a brief history  of the  development of the
topic  and  then  considers  the subjects  of  quasars in   groups and
clusters   of galaxies, quasars  in  superclusters,  and, finally, the
distribution in space of quasars themselves.

  The first observations that   low redshift quasars  have  resolvable
structures around  their nuclei  prompted a  number of  studies on the
nature  of the surrounding  "fuzz."  During the  course of  that work,
which provided evidence for the cosmological nature of the red-shifts,
it was noted that low-redshift  quasars were frequently accompanied by
nearby galaxies, some  of which  appeared  to be interacting with  the
quasar.   Although such quasars  were not  found  in rich  clusters of
galaxies,  they  often did  occur  in  groups of galaxies. Subsequent,
extensive studies of the environment of quasars at successively larger
redshifts  form the basis  of much  of  our knowledge about quasars in
groups and clusters of galaxies. Over the  same interval much work was
being done on the  nature and evolution  of galaxies, galaxy clusters,
and the large-scale structure of the universe, and it has become clear
that  the mechanisms that trigger  the ignition of quasars are closely
related to fundamental   events     in galaxy groups     and  clusters
themselves.

  As work continued on quasar surveys,  the possibility arose of using
quasars  themselves  as tracers of  the  structure of  the universe at
distances significantly beyond those  attainable with normal galaxies.
The light travel time to such a quasar covers more than three-quarters
of the age of  the universe, thereby offering   a chance to  study the
evolution of structure over  an  interval that  could be  decisive for
choosing among different theories.  At the same  time the first groups
of quasars at  redshift 2 were found, and  their possible  relation to
superclusters was noted.  Subsequently the  first detection of  quasar
clustering at small scales  was made, a  topic which is currently very
lively and  not completely settled.  The addition of large, systematic
surveys  for quasars has  made it possible  to address the question in
detail.

  Observationally, work on galaxies in the vicinity of quasars is made
difficult  by  the great disparity of    their luminosities and forms:
Quasars have bright, stellar-appearing  nuclei that can easily be  100
times brighter than galaxies,  which are spatially extended and  often
faint compared to the brightness of the night  sky. The development of
modern  detectors such as  CCDs  (charge coupled devices), which  have
high quantum efficiency, excellent stability,  and wide dynamic range,
has been   key to advances in  the  field. Even  so, with  the present
generation of  telescopes it  is difficult  to pursue  the problem  at
redshifts  above 0.6.  Therefore, in view   of the strong evolution of
quasars toward  higher  redshifts, it  must  be  recognized  that  the
available sample for study may not show all the phenomena that occur.

  Throughout this entry redshifts are   used as a measure of  distance
and cosmic epoch. Distance increases with  redshift, as does look-back
time, which is defined as the travel time  for light from an object at
a given redshift divided by the elapsed time since the Big Bang (for a
Friedmann cosmology). For  reference,  if the  values of  H*=50 km s**
Mpc** and **=0.5 are adopted, then the values in Table 1 are obtained.

QUASARS IN GROUPS AND CLUSTERS OF GALAXIES

We now know that  both  quasars which  are sources of  radio  emission
(radio-loud quasars) and those which are not (radio-quiet quasars) are
not found in  rich clusters of galaxies  for redshifts less than  0.5.
However,  they do  occur in areas   where  the density  of galaxies is
enhanced by a  factor  of 2-3  over  the  density of  the  background,
consistent with earlier  remarks  on the  tendency of such  quasars to
have companions or  occur in groups.  For redshifts less than 0.5,  no
difference in the environment of radio-loud and radio-quiet quasars is
apparent.

  However, a  striking result  is     that radio loud  quasars    with
0.5<z<0.65 occur in regions where galaxy density is eight times larger
than average, with half of  them occurring in  clusters of galaxies of
Abell  richness  1 or   more (clusters with    50-79 members  within 2
magnitudes of  the third  brightest  member). The implication  is that
radio   quasars in   clusters    of galaxies evolve  strongly  between
red-shifts 0.4  and 0.6 and have dimmed  by several magnitudes  by the
epoch corresponding to redshift 0.4. Also, the galaxies themselves are
brighter   than normal in    the  clusters containing the   radio-loud
quasars,   an indication  that   they   have also   evolved.  However,
radio-loud quasars in less   dense environments have not  evolved  and
therefore dominate the population at redshifts less than 0.5.

  The  result implies  that the  environment has   a strong  effect on
quasars  and  that     conditions   in rich clusters     have  changed
significantly  between redshifts of 0.6  and  0.4. It subsequently was
found that some faint, radio-loud  quasars occur in Abell 1  clusters,
consistent with the  idea. It  appears  that the rate   of decay is  1
magnitude (a factor  of 2.5) per  0.5 Gyr.  Subsequently, it has  been
found  that low-level activity in  radio  galaxies of low redshift can
occur in rich  clusters;  this is  consistent  with a rapid fading  of
quasar activity with time.

  The  addition of new data  on faint  quasars  at low redshifts shows
that radio-loud quasars do occur in regions of enhanced galaxy density
at all accessible redshifts.  This  is consistent with the  hypothesis
that radio-loud  quasars occur in  elliptical galaxies and radio-quiet
quasars   in spirals. An  unresolved possibility  at   present is that
radio-quiet quasars may occur on the outskirts of rich clusters.

QUASARS IN SUPERCLUSTERS; GROUPS OF QUASARS

At redshifts larger than  0.6 the space  density of quasars  increases
significantly but the  detection of accompanying galaxies becomes very
difficult. Therefore, the topic of quasars, in superclusters generally
covers  the finding of groups  of  quasars with similar redshifts that
have  the  dimensions of  superclusters.  During  the  course of large
surveys  for quasars, several such groups   have been found. They have
dimensions ranging  from the size of galaxy  clusters, 10 Mpc or less,
to greater than 100 Mpc.

The interest in quasar groups is at least twofold:

   1. The spatial distribution of quasars and their relation to galaxies
      yields information on the processes and conditions leading
      to the formation of quasars. Quasars can be observed at high
      enough redshift to cover the evolution of such effects over a
      substantial part of the age of the universe.

   2. To the extent that quasars trace the distribution of matter in
      the universe, they give us the only available information on
      how the large-scale structure of the universe itself has evolved.
      This information is of critical importance to cosmology and
      to theories of galaxy formation.

DISTRIBUTION OF QUASARS IN SPACE

Quasar surveys have reached the point that analyses of their own space
distribution at high   redshift can now be   done in analogy  with the
early  studies of the   galaxy   distribution. These programs are    a
generalization of  searches for groups of quasars  and are intended to
probe the basic question  of how the population of  quasars as a whole
is distributed in space.

  The  first analyses of quasar surveys  showed no evidence for quasar
clustering  and  were  consistent    with quasars   being  distributed
uniformly at random  in  space. Recognized shortcomings of   the first
work, however,  were  that the samples  contained only  a few  hundred
objects;  they were based on  surveys whose selection effects were not
necessarily well known; and  the  surveys did  not reach quite   faint
enough to  attain the density of quasars  needed to probe structure on
small scales.  Therefore, they did  not place very stringent limits on
the presence or absence of clustering. However, the surveys all tended
to contain intriguing  groups of the  type mentioned in  the preceding
section.

  The  first   positive detection of  quasar  clustering  came from an
ingenious test applied to  the inhomogeneous but  large sample of  all
known quasars. It provided evidence  for quasars being clustered on  a
scale of 10 Mpc. Subsequent analyses have continued to show clustering
on  this scale, with some  indication that it is  present at z<1.5 but
not at larger redshifts. The amplitude is intermediate between that of
galaxies and clusters of galaxies.  Some researchers do not agree that
the result    is generally applicable  to the   quasar population as a
whole, although  they  recognize  that  there are  sizable  individual
groups of quasars.

  Attempts have also been made to analyze  the distribution of quasars
on the  very large scale  by  considering  their  occurrence over  the
entire sky. Some  intriuging non-uniformities have  been noted, but it
is difficult to  establish their reality, for  lack of  a sufficiently
well-calibrated and homogeneous survey for quasars over the whole sky.

SUMMARY

The subject of the  environments of quasars and the  three-dimensional
distribution of quasars in space is  new enough for  few results to be
definite. At  the  same time, the results  to  date  show  that quasar
environment are  of great interest to   determining how quasars evolve
and under  what conditions  and also  promise to  tell us about galaxy
evolution. At  large redshifts, the  clustering properties  of quasars
provide our only  information on the   distribution of matter.  Large,
systematic surveys for quasars are  crucial to future progress in  the
field.  (Preliminary   x-ray  observations from  the   ROSAT satellite
provided suggestions of quasar clustering, reported in 1991-Ed.)

     Additional Reading

Hutchings, J.B.(1983). QSOs: Recent clues to their nature.
   Publ. Astron. Soc. Pacific 95 799.

Osmer, P.S.(1981). The three-dimensional distribution of quasars
   in the CTIO surveys. Ap. J. 247 762.

Powell, C.S.(1991). X-ray riddle.
   Scientific American 264 (No. 3) 26.

Shaver, P.A.(1988). Quasar clustering and the evolution of structure.
   In Large Scale Structures of the Universe, IAU Symposium
   130, J. Audouze et al., eds., Kluwer, Dordrecht, p. 359.

Yee, H.K.C.(1990). The evolution of galaxies and galaxy clusters
   associated with quasars. In The Evolution of the Universe of
   Galaxies: The Edwin Hubble Symposium, R.G. Kron, ed.
   Astronomical Society of the Pacific Conference Series,
   vol. 10, San Francisco.