ESA’s X-ray observatory, XMM-Newton, has for the first timeallowed scientists to study in detail the formation history of galaxyclusters, not only with single arbitrarily selected objects, but with acomplete representative sample of clusters.
Knowing how these massive objects formed is a key to understanding the past and future of the Universe.
Scientistscurrently base their well-founded picture of cosmic evolution on amodel of structure formation where small structures form first andthese then make up larger astronomical objects.
Galaxy clustersare the largest and most recently formed objects in the known Universe,and they have many properties that make them great astrophysical‘laboratories’. For example, they are important witnesses of thestructure formation process and important ‘probes’ to test cosmologicalmodels.
To successfully test such cosmological models, we musthave a good observational understanding of the dynamical structure ofthe individual galaxy clusters from representative cluster samples.
Forexample, we need to know how many clusters are well evolved. We alsoneed to know which clusters have experienced a recent substantialgravitational accretion of mass, and which clusters are in a stage ofcollision and merging. In addition, a precise cluster mass measurement,performed with the same XMM-Newton data, is also a necessaryprerequisite for quantitative cosmological studies.
The mosteasily visible part of galaxy clusters, i.e. the stars in all thegalaxies, make up only a small fraction of the total of what makes upthe cluster. Most of the observable matter of the cluster is composedof a hot gas (10-100 million degrees) trapped by the gravitationalpotential force of the cluster. This gas is completely invisible tohuman eyes, but because of its temperature, it is visible by its X-rayemission.
This is where XMM-Newton comes in. With itsunprecedented photon-collecting power and capability of spatiallyresolved spectroscopy, XMM-Newton has enabled scientists to performthese studies so effectively that not only single objects, but alsowhole representative samples can be studied routinely.
XMM-Newtonproduces a combination of X-ray images (in different X-ray energybands, which can be thought of as different X-ray ‘colours’), and makesspectroscopic measurements of different regions in the cluster.
Whilethe image brightness gives information on the gas density in thecluster, the colours and spectra provide an indication of the cluster’sinternal gas temperature. From the temperature and densitydistribution, the physically very important parameters of pressure and‘entropy’ can be also derived. Entropy is a measure of the heating andcooling history of a physical system.
The accompanying threeimages illustrate the use of entropy distribution in the ‘X-rayluminous’ gas as a way of identifying various physical processes.Entropy has the unique property of decreasing with radiative cooling,increasing due to heating processes, but staying constant withcompression or expansion under energy conservation.
The latterensures that a ‘fossil record’ of any heating or cooling is kept evenif the gas subsequently changes its pressure adiabatically (underenergy conservation).
These examples are drawn from theREFLEX-DXL sample, a statistically complete sample of some of the mostX-ray luminous clusters found in the ROSAT All-Sky Survey. ROSAT was anX-ray observatory developed in the 1990s in co-operation betweenGermany, USA and UK.
The images provide views of the entropydistribution coded in colour where the values increase from blue,green, yellow to red and white.
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