Ph.D. Thesis

Substructure, Dynamics and Evolution in

Clusters of Galaxies

by Christina M. Bird

Under the supervision of Professor John M. Dickey

ABSTRACT

 
Most models for the formation of large scale structure imply that clusters of galaxies are currently forming. Because the Hubble time is on the order of a few dynamical timescales for clusters, there is some hope that their structure will retain signatures of their formation and initial conditions.
 
In order to objectively identify the different sorts of substructure which may exist in clusters of galaxies at the present time, I present a variety of statistical diagnostics which measure deviations of cluster galaxy velocities and positions form those expected in a relaxed system. These tests are applied to a database of rich clusters with at least 50 measured redshifts. Depending on the diagnostic considered, 30-70% of all well-studied clusters possess significant substructure. Only one system in the cluster database, A496, does not have any significant deviations from the structure expected for a relaxed, dynamically-evolved cluster.
 
Once the importance of substructure in any particular cluster has been assessed, a maximum likelihood technique is used to assign individual galaxies to their host subclusters.
 
These statistical techniques are applied to the study of central galaxy peculiar velocities. Formation models for central dominant galaxies require them to be preferentially located at the bottom of their cluster potential well. Without correcting for substructure, 50% of the cD-type systems in the cluster database have central galaxies which are displaced from their potential minimum (as defined by the cluster average velocity). These systems are much more likely to exhibit correlations between galaxy velocities and positions that clusters in which the central galaxy is at rest, providing evidence that substructure may be responsible for the high velocity offsets. If dynamical models which include subcluster assignments are used to determine the peculiar velocities, only 20% of the cD-type clusters have significant velocity offsets.

Master's Thesis

Dark Matter and Dynamics in the Hercules Cluster

by Christina M. Bird

Under the supervision of Professor John M. Dickey

ABSTRACT

We present new 21-cm observations of fain (15.7 <mpg < 16.5) spiral galaxies in the Abell cluster 2151. These results, when combines with the large body of velocities available i n the literature for Hercules, permit us to study the dynamics throughout the cluster core, out to a projected radius of 1.8 Mpc/h. We calculate the dynamical mass of Hercules using 3 different but related methods. Two versions of the virial theorem, each describing slightly different distributions of the gravitating matter (whether luminous or dark) has been performed: the luminosity-weighted virial theorem assumes that the majority of the gravitational potential is contributed by the galaxies themselves; and the number-weighted virial theorem considers the galaxies as test particles for an underlying mass distribution (with scale lengths for the gravitational potential determined from the galaxy distribution); and the projected mass estimator, which is more stable that the virial theorem to statistical uncertainties in the three dimensional distribution of the galaxies, but still presumes that they are effective tracers of the mass.
 
We investigate the importance of subclustering in A2151 using the statistical test of Dressler and Shectman (1988). We also study the effects of the detected substructure on the dynamical mass determination. The highly clumpy distribution of galaxies (in both physical and velocity space) is interpreted as a sign that the galaxies in the cluster have not reached dynamical equilibrium in the gravitational potential, which means that the virial theorem mass estimate (or any mass determined under the assumption of dynamical equilibrium) is prone to significant errors. To estimate these, we perform the same analysis on the Virgo Cluster, and compare our results to the more-accurate mass models of Hoffman and Salpeter (1982) and Tully and Shaya (1984). This test shows that the substructure introduces a 20-30% error in the uncorrected dynamical mass determination. Finally we us the 21-cm line widths to estimate the minimum total mass in the cluster which is contained within the HI radii of the cluster galaxies. This number may be compared with the dynamical mass and used to separate the contribution of dark matter inside and outside the HI envelopes of galaxies. The projected mass estimator of the cluster, in conjunction with the line widths, is also used to study the fraction of DM which cannot be associated with individual galaxies.