A particularly nice example of a cold front in the cluster Abell 2034.

The most favoured model for how galaxies form and cluster in the universe – the “Lambda Cold Dark Matter” model – predicts that the largest and most massive structures, such as clusters of galaxies, form in a hierarchical manner. As such, many of these systems are expected to have recently undergone major mergers, where cluster-sized mass concentrations have coalesced to form an even larger system, a process which is likely to be both violent and have dramatic consequences for the hot intracluster medium (ICM) and galaxies that inhabit these systems. In order to test and investigate this scenario observationally, we require an easily identified signpost that indicates a cluster of galaxies has recently undergone a major merger. While a major cluster-cluster merger should present itself in a number of different ways e.g.; multiple peaks in the spatial and redshift distributions of the cluster galaxies, and shock fronts and complex morphology in the ICM as revealed in X-ray images, such signatures have proved to be both unreliable and difficult to detect and interpret.

We have been exploring the use of a new signpost which exploits the unprecedented spatial resolution and sensitivity of the Chandra X-ray Observatory. The ability of this space-based X-ray telescope to image the ICM at new levels of resolution, has brought with it new discoveries, including that of “cold fronts”. Cold fronts in the ICM are analogous to those observed on earth, albeit at temperatures ~4 orders of magnitude higher! They are bodies of cool gas moving through the hot, ambient ICM, forming a contact discontinuity at their point of intersection. These cold fronts are thought to be signs of a recent merger, although simulations predict different species of cold fronts caused by different physical triggering mechanisms.

We have selected a sample of “cold front” clusters from the Chandra data archive, and are using them to undertake a multi-wavelength study of the effects of mergers on the ICM and on the galaxies residing in this environment at multiple wavelengths. We hope to discover how effective a cold front is as a “merge-o-meter” and find observational evidence for the different types of cold fronts observed in the simulations.

Matt Owers and Warrick Couch