Clusters of galaxies, the largest known gravitationally-bound objects, are the knots in the cosmic web of structure that suffuses the Universe. Theoretical models of the Universe make predictions about the numbers, distributions and properties of galaxy clusters. By comparing these predictions with observations, scientists can test and improve models of the universe. The most powerful way of doing this is to measure the masses of galaxy clusters, particularly those in the distant universe. However, it is extremely difficult to weigh galaxy clusters.

A team of researchers, led by Ben Maughan of the Harvard-Smithsonian Center for Astrophysics, observed 11 extremely distant galaxy clusters with the ESA's XMM-Newton and NASA's Chandra X-ray Observatory. The X-ray data allowed the scientists to measure the temperatures and luminosities of the gas in the clusters, and infer their total masses.

These measurements were then used to test whether the galaxy clusters are simply scaled versions of each other -- a condition known as being "strongly self-similar." This is an important characteristic to be able to identify if astronomers hope to get the true weights of galaxy clusters.

"For example, a chocolate bar is strongly self-similar. If you shrank a king-size bar to a fun-size bar, they would be identical versions of each other but just different sizes," said Maughan. "However, if you shrank a castle to the size of a bungalow, they would be very different structures despite being the same size. This means that they are not strongly self-similar objects."

Another possible type of relationship between clusters is what scientists call "weakly self-similar". In this case, galaxy clusters in the distant universe and those nearby are almost identical to each other, but not exactly. A factor to account for the expansion of the universe must be added.

While astronomers have known for some time that galaxy clusters are not strongly self-similar, whether or not clusers are weakly self-similar is still an open question. Understanding this will make it much easier to measure the masses of distant galaxy clusters.

These new X-ray results show that as long as astronomers take into account the continuous expansion of the universe, then galaxy clusters are, in fact, weakly self-similar, and the local scaling relations hold true for these very distant clusters.

"Our results mean that weighing distant galaxy clusters could become as easy as converting from Fahrenheit to Celsius!," said Maughan. "This will help to answer important questions about the nature and structure of the universe."

Additional material:

The image below shows computer simulation of a large volume of the universe by the Virgo consortium (Jenkins et al. 1998). An XMM-Newton X-ray image of a real galaxy cluster from our study is superimposed to illustrate the formation of galaxy clusters in the densest parts of the universe.

Because it is so difficult to measure the masses of galaxy clusters important future work will involve combining different methods of measuring cluster masses, such as X-ray and gravitational lensing measurements, and using those results to improve the reliability of the scaling relations.

For other recent research in this area, including similar results on the scaling relations of distant galaxy clusters, see the following publications: