My thesis project has been motivated by the lack of a systematic study of galaxy cluster mergers. Typically, clusters are studied numerically in two fashions: cosmological simulations and idealized binary mergers. Although the former are thought to be more faithful representations of mergers, typically they either do not incorporate gas dynamics in the calculations (i.e. DM-only simulations) or the fail to resolve adequately the merging halos. Binary merger simulations offer a more controlled environment for the study of the merging process. However, thus far, these simulations have not covered an significant volume in parameter-space.
My work is geared toward alleviating this situation, in a way that complements the results of cosmological simulations. My simulations cover the range 1013—1015 Msol for mass of the primary cluster, one order of magnitude in the ratio of masses of the merging clusters and one order of magnitude in the impact parameter (i.e., the orbital angular momentum).
For the initial conditions I use clusters that are in good agreement with observations of relaxed clusters. These models have been constructed by requiring that clusters are in hydrostatic and convective equilibrium, and allow for a varying gas mass fraction, Fgas(M). I have tested their reliability by producing density and temperature profiles for the clusters of Viklhnin et al. (2006) (V06); my models are within 20% of these detailed observations. Also, the global properties of these models are in good agreement with the observed scaling relations. The image on the right compares the LX-YX properties of the V06 clusters reproduced by this method with the observed relation by Maughan (2007).
The simulations are done with the N-body/SPH code Gadget-2. Currently, I am in the process of completing the library of merger simulations on Teragrid facilities (30,000 CPU hours award; 130,000 CPU hours total) and a 24-node Beowulf Cluster owned by the Astronomy Department, UVa. Examples of these simulations are shown below.
|Merger b/w a 1014 Msol and a 1013 Msol clusters with high impact parameter, viewed 2 Gyr before core-crossing.||Cross-section movie (22 MB) of a merger b/w a 1014 and a 1013 Msol clusters with high impact parameter, evolved for 15 Gyr.||Head-on collision b/w two 1015 Msol clusters, viewed 0.5 Gyr before core-crossing.|
Our interest focuses on the temporal variation of the X-ray luminosity and the SZ signal of the merging clusters. Once these variations are known, our results will be combined with merger trees to evalulate the bias that mergers may induce in the value of cosmological parameters, as obtained by Dark Energy Surveys that use clusters as tracers of the cosmic expansion.
Upon completion of this project, our simulation data will become
publicly available to aid the processing and planning of observations
of merging galaxy clusters.
This research is partially funded by NASA under grant awarded
via the Chandra
My experience in X-ray astrophysics began with the analysis of a 41 ksec Chandra observation of the merging cluster Abell 2065, under the guidance of Dr. Craig Sarazin (Chatzikos, Sarazin & Kempner, 2006). Previous ROSAT and ASCA observations suggested that both cores survived the merger.
However, only one peak was present in the higher spatial resolution Chandra image. The second feature to the north was not, in fact, associated with the northern galaxy, rather it proved to be a trail of cool gas behind the main cD. The gas associated with the southern cD was found to be slightly displaced from the position of the galaxy, forming a cold front. Numerical simulations (e.g., Heinz et al. 2003) suggest that the cold front and the tail to the north have been formed by ram-pressure stripping of the southern cool core during the merger. We also confirmed the presence of a surface brightness discontinuity situated at a projected distance of ~150 kpc southeast of the main cD. The discontinuity has a Mach number of 1.7±0.25, however its nature (shock front or cold front) cannot be confirmed from the existing data.
We argued that Abell 2065 is actually undergoing a minor merger.
The most massive cluster to the south has disrupted the intracluster
medium (ICM) of the secondary halo, and is now moving southeast.
Based on numerical simulations, we proposed that the system had
undergone core-crossing a few hundred Myr ago, probably for the
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