Observation of high-temperature gases in the Virgo Galaxy Cluster
Generally, a substance radiates more high-energy electromagnetic waves as its temperature increases. High-temperature gases with temperatures of several to tens of million deg. C radiate strong X-rays. These X-rays can be observed. Further, under such high-temperature states, oxygen, iron, etc., are heavily ionized to strongly emit bright lines with specific energy. We can ascertain the amount of each element from the whole spectrum shape and the intensity of the bright lines.
The main image shows an X-ray image of the center of the Virgo Galaxy Cluster, the closest galaxy cluster to the earth. Fig. 1 shows the X-ray spectrum emitted from the central region. The bright lines from magnesium (Mg), silicon (Si), sulfur (S), argon (Ar), calcium (Ca) and nickel (Ni) as well as oxygen (O) and iron (Fe) are clearly detected. By identifying the amount of each element based on the bright-line intensity, we are now investigating: (1) the type of supernova explosion that synthesized the elements, i.e., the type and number of stars present; (2) whether or not the elements contained in the high-temperature gases have the same origins as those of nearby stars in the Milky Way or the sun.
In the center of a galaxy cluster, such as the Virgo Galaxy Cluster, the important factor is the contribution of elements synthesized by gases recently emitted from stars in the elliptical galaxy located at the center of the cluster and by recent supernova explosions originating from lightweight stars. Meanwhile, outside the galaxy-cluster center, the important factor is the contribution of elements synthesized over long periods by various supernova explosions from various galaxies. Fig. 2 shows the distribution of oxygen, magnesium, silicon, and iron contained in the high-temperature gases in the center of the Virgo Galaxy Cluster. The composition ratio of all these elements gradually increases toward the center because of the contribution of the elliptical galaxy located in the center. The ratios of silicon and iron are almost the same as those of the sun and their composition ratios gradually increase in the same way toward the cluster’s center. In contrast, the composition ratios of oxygen and magnesium are smaller than silicon and iron. The ratio of oxygen in particular is half or less than that of silicon and iron.
Fig. 3 shows a comparison of the ratio of magnesium/oxygen in high-temperature gases and that of stars in the galactic system. These two elements are synthesized only by the supernova explosions occurring when a heavy star dies. Fig. 3 shows that the ratio of magnesium and oxygen contained in high-temperature gases is correspondent to that of stars in the galactic system. This fact means that there is no major difference in element synthesis by supernova explosions, whether in the galactic system or in elliptical galaxies, at least in the ratio of these two elements.