The Cluster and Large Scale Environments of Quasars at z < 0.9.
Doctoral thesis, University of Central Lancashire.
We present an investigation into the environments of quasars with respect to galaxy clus-
ters, and environment evolution with redshift and luminosity. The positions of quasars
with respect to clusters have been studied using cluster and quasar catalogues available,
covering the redshift range 0.2 < z < 1.2. The 2D projected separations and the 3D
separations have been found and the orientation of the quasar with respect to the major
axis of the closest cluster calculated, introducing new information to previous work.
The positions of quasars with respect to clusters of galaxies will give an indication
of the large scale environment of quasars and potentially clues as to which formation
mechanisms are likely to dominate at various redshifts. For example, galaxy mergers
are most likely to occur in galaxy group environments and will create luminous quasars.
Galaxy harassment is more likely to occur on the outskirts of galaxy clusters and create
lower luminosity AGN. Secular processes such as bar instability can also create AGN and
are likely to be the cause of nuclear activity in isolated galaxies. The aim of this work is
to study the large scale environment over a large redshift range and study the evolution
as well as any change in environment with quasar luminosity and redshift. Another aim
of this work is to study the orientation of a quasar with respect to a galaxy cluster. If
galaxy clusters lie orientated along filaments, the position of a quasar with respect to a
cluster will give an indication as to where quasars lie with respect to the filament and
therefore the large scale structure.
There is a deficit of quasars lying close to cluster centres for 0.4 < z < 0.8, indicating
a preference for less dense environments, in agreement with previous work. Studying
the separations as a function of cluster richness, there was a change in quasars lying
closer to poorer clusters for z < 0.2 (Lietzen et al. 2009) to lying closer to richer clusters
for 0.2 < z < 0.4, though more clusters at low redshifts will be needed to confirm
this. There is no obvious relation between the orientation angle between a quasar and
the major axis of the closest galaxy cluster and 2D projected separations. Using faint
(Mr > −23.0 mag) and bright (Mr < −23.0 mag) quasars, there is no difference between
the two magnitude samples for the 2D separations or the cluster richness, in contrast
to Strand et al. (2008) who found brighter quasars lying in denser environments than
dimmer quasars. These is no change with redshift (over 0 < z < 1.2) in the positions of
the quasars with respect to the cluster or the cluster richness as a function of absolute
quasar magnitude. There is also no preferred orientation between the quasar and the
cluster major axis for bright or faint quasars.
Spectra of a selection of 680 star forming galaxies, red galaxies, and AGN were taken by
Luis Campusano and Ilona S¨ochting and 515 redshifts calculated. Though few of these
galaxies turned out to be cluster members as was originally intended, it was possible
to use these galaxies to study the environments of quasars with respect to star-forming
galaxies and galaxy clusters. The objects were classified (33 classed as AGN), and star
formation rates calculated and compared. Three AGN and 10 star forming galaxies lie
at the same redshift (z = 0.29) as three galaxy clusters. The three galaxy clusters have
the same orientation angle and may be part of a filament along with the star forming
galaxies and AGN. Further study will investigate the relation between AGN positions
and filaments of structure.
A sample of quasar spectra taken by Lutz Haberzettl using Hectospec on the MMT were
taken to increase the number of quasars used in this study. However, when studying
the spectra, a number of high redshift quasars showed evidence of ultra-strong UV Feii
emission in their spectra. The redshifts of these quasars were too high to be included
in the main body of the study. However, a significantly large number of ultra-strong
UV Feii emitting quasars have been found in the direction of three LQGs in the redshift
range 1.1 < z < 1.6, including the Clowes-Campusano Large Quasar Group (CCLQG).
Ly� fluorescence can increase the UV Feii emission. However, Ly� emission from other
quasars was found to be negligible compared to emission from the quasar’s central source.
Though there has been no previous indication that the LQG environment is unique, the
high level of iron emission may indicate a difference in environment. Plans for future
work based on these results are outlined.
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