Galaxy Clusters
Please note that this page is still under construction
Overview
Clusters of galaxies have become a cornerstone of the study of large-scale structure. In a Universe dominated by cold dark matter (CDM), clusters are the largest and latest structures to have collapsed and reached equilibrium; large enough to contain a near-cosmic mixture of matter and late enough to still retain cosmological information. Multi-wavelength observations are now routinely mapping the distribution of galaxies, intracluster medium (X-ray and Sunyaev-Zel'dovich (SZ) effect) and dark matter (strong and weak gravitational lensing), thus offering direct confrontation with the CDM paradigm and galaxy formation models. Present and upcoming X-ray and SZ surveys promise results that will position clusters as a competitive probe of dark energy, should the systematics associated with mass-observable calibration be sufficiently well constrained.
At JBCA, we have an active and varied research programme into galaxy clusters, ranging from numerical models and simulations to participation in large cluster surveys. Below, we highlight the key areas of activity.
Gas Dynamic Simulations
Scott Kay
Zooming into a merging galaxy cluster from its surrounding large-scale
structure. Taken from the z=0 snapshot of the CLEF simulation. The
length of the full box is 200 Mpc/h.
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Clusters are dynamically young systems and contain large amounts of substructure as a result of the continual accretion of matter from the large-scale structure around them. The intracluster medium is therefore quite complex, frequently showing evidence of turbulence, shocks, cavities and contact discontinuities termed cold fronts. The effects of galaxies on the intracluster gas, through both stellar and nuclear activity, only serves to make the problem more complicated. To investigate the detailed internal structure of clusters, we run and analyse large numerical simulations of clusters.
We are involved in numerous cluster simulation programmes. A recent example is the CLEF simulation, a result of a collaboration between UK and French researchers, and run on a powerful supercomputer at CINES in Montpellier. An image taken from the CLEF simulation is shown on the right. The simulation was one of the first of its size to follow radiative gas dynamics as well as containing a simple prescription for strong energy feedback from galaxies. As a result, many of the gross properties of the low redshift cluster population could be matched and the distribution of structure for around 100 clusters was investigated. A particularly interesting prediction from the simulation was the disappearance of the cool core clusters by z=1, leading to a systematic decrease in the scatter of the X-ray luminosity-temperature relation with redshift. See Kay et al., 2007, MNRAS, 377, 317 for more details.
The large-scale gas distribution in the Millennium Gas simulations. Here, the box
is 500 Mpc/h on a side.
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Manchester is also a UK node of the Virgo supercomputing consortium and most of our activity takes place within Virgo. We are currently involved in a follow-up project to the ten billion particle Millennium Simulation, known as the Millennium Gas project. This is being led by Frazer Pearce at the University of Nottingham, in collaboration with ourselves and Peter Thomas at the University of Sussex. The advantage over CLEF is twofold: a simulation volume that is 15 times larger means that we simulate many more clusters, especially the larger ones (at z=0 we have around 2,000 clusters!) Secondly, we are running several simulations, each with different assumptions for the gas heating and cooling processes. This allows us to compare the effects of uniform heating at high redshift (preheating) and also in high density regions at all redshifts (feedback) to a model with no extra heating or cooling at all (non-radiative).
We are also interested in performing higher resolution simulations of individual clusters. This is achieved using the resimulation technique, where the resolution of the surrounding large-scale structure is degraded and serves only to provide the tidal force field; instead the computational effort goes into following the cluster with much more detail than in simulations with uniform resolution. This allows us to follow substructure with larger dynamic range and investigate detailed structure in the hot gas and its interaction with the cluster galaxies.
Click the image to see a movie of a forming galaxy cluster within a
cosmological simulation. Colours indicate the temperature of the
intracluster gas (1-100 million K) and contours the density of gas.
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Sky maps of clusters and large-scale structure
Richard Battye, Scott Kay
Clusters can be readily identified in the X-ray with little contamination from background sources, due to the rapid decrease in X-ray surface brightness with redshift. Newer techniques for observing clusters, exploiting gravitational lensing and Sunyaev-Zel'dovich effects, measure the integrated signal along the line of sight, however. To address the effects of projection from large-scale structure, we are construcing sky maps of the cluster population, using both analytical and simulation methods. Analytical models allow us to rapidly generate maps for a large range of cosmological models, enabling detailed studies of the effects of cosmological parameters to be performed. Simulations, such as the Millennium Gas simulations above, are restricted to several models at best, but allow us to focus on the detailed effects of unvirialised structures, such as the contribution of substructures and the intergalactic medium to the SZ effect.
Cluster Surveys
Richard Battye, Scott Kay
New X-ray and SZ surveys promise to find many new clusters at high redshift (beyond z=1), of which we currently know very little about. High redshift clusters are very interesting for two reasons. Firstly, the number and distribution of clusters at high redshift potentially places strong constraints on cosmological parameters such as the equation of state of dark energy. Secondly, the amount of star formation and feedback activity is known to increase with redshift so we ought to learn more about the effects of these processes on the intracluster gas by studying high redshift clusters.
X-ray (blue haze) and optical/infrared composite image of the most distant
known X-ray cluster in the Universe, at z=1.45. From the XCS.
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At Manchester we are currently involved in two cluster surveys. The first is known as the XMM Cluster Survey, a serendipitous search for clusters in archival (public) XMM X-ray data. It is expected that 500 square degrees will have been searched by the end of the survey, potentially finding more than a thousand clusters, most of them new detections. An extensive follow-up programme is also being performed at the NOAO to determine photometric redshifts for some of the clusters. Some high redshift candidates are also being followed up with 8-10m class telescopes such as Keck; the most famous example being the highest-redshift known X-ray cluster at z=1.45, shown on the left. Scott Kay is involved with the XCS, providing data from cosmological simulations to investigate the effects of cluster structure and substructure on the detection probability as well as modelling the evolution of the luminosity-temperature relation with redshift.
We are also involved with the proposed Planck SZ cluster survey. Planck will perform an all-sky survey in several frequency bands, with the expectation of detecting as many as 10,000 SZ clusters.
