TOP > Report & Column > The Forefront of Space Science > 2015 > The chemical composition of the Universe on the largest scales
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Actually, perhaps counterintuitively, the answer to these questions is best found not by looking at the stars themselves, but rather looking at the intergalactic space. That is because most of the normal matter in the universe, and thus also most of the metals, are presently not contained in stars, but rather in a very hot, diffuse gas that fills the space between galaxies. This is especially true for “clusters of galaxiesE the parts of the Universe where the galaxies are packed closest together. In clusters of galaxies, about 90% of all the normal matter is a diffuse gas known as “intra-cluster medium,Ewhich is so hot that it shines in X-ray light. The chemical composition of this gas can be measured using X-ray spectroscopy: essentially, different chemical elements in the intra-cluster medium like to emit light at specific energies; based on the wavelength of the spectral emission line, one can identify the ion responsible for producing it, and based on the strength of the line, one can usually infer how abundant that ion is. I’ve found this idea fascinating ever since the first year of my PhD: X-raying the chemical content of our Universe. However, back then, almost 10 years ago, it was very hard to obtain reliable measurements of the metal abundances except for the very densest, brightest parts of the intra-cluster medium Ethe central few hundred thousand light years, which sounds like a lot but in fact represents only about a thousandth part of the total volume of a typical galaxy cluster. There were some interesting hints that the chemical composition towards larger radii was changing, namely, that the central parts contained more thermonuclear supernova products like iron, while the outskirts of galaxy clusters were enriched just by core collapse supernovae. But it was very hard to tell for sure, since at large radii from the centres, there were very few X-ray photons and a lot of background noise, and different articles often came to different scientific conclusions. JAXA’s Suzaku X-ray satellite dedicated a lot of observing time, collecting data over many weeks, to address this question. Because the background noise of Suzaku is lower than other X-ray detectors that are currently operational, it was able to provide more reliable measurements of the chemical enrichment patterns in the faint, outer regions of clusters of galaxies than was possible ever before. Some of the first results, based on observations of the nearest, brightest system, the Perseus Cluster, showed a remarkably uniform distribution of the abundance of iron in the intra-cluster medium on large scales. This would suggest that the outskirts of clusters of galaxies were not enriched only by core collapse supernovae, as initially hinted at by previous observations, but rather that thermonuclear supernovae must have also played a role. However, in order to really tell what types of supernovae contributed to the metals in the intracluster medium on these very large scales, it was necessary to measure not only the abundance of a thermonuclear supernova product such as iron, but also that of a chemical element predominantly produced by core collapse supernovae, so that the two abundances could be directly compared. This was not possible for the Perseus Cluster because, at the average temperature of the gas in this system, all lines from chemical elements other than iron are very weak and hard to detect. Observations of a galaxy cluster with a lower average temperature would be better for such measurements, because the lines from elements typically produced by core collapse supernovae are comparatively stronger than in the Perseus Cluster.
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