Associate Professor
Division of Plasma Astrophysics
Galaxy cluster halo within a large-scale structure
The Milky Way galaxy is located near the edge of a group or cluster of galaxies called the Virgo Supercluster. Galaxy clusters have dozens to thousands of galaxies in them. They are the largest structures in the Universe that are self-bound by their own gravity. They extend roughly 10 million light years in diameter, and their masses are equivalent to 100 to 1,000 trillion times our Sun. The majority of their mass exists as dark matter, which occupies 80% of the cluster mass. The rest are stars and high-temperature plasma that are bounded by the enormous gravitational potential of the dark matter, and each is estimated to contribute between 10% and 20% of the total mass.
Dark matter is deeply involved in the formation of galaxy clusters. The small inital density fluctuations of dark matter in the early universe gradually grew through gravitational interactions and formed anywhere in the Universe enormous structures called dark matter halos , which are bounded by their own gravity. The halos began forming with small masses that gradually merged each other and grew larger over time. (See Figure 1.) It is thought that galaxies were formed in the dark matter halos, and the gas within the halos cooled and contracted, resulting in the birth of stars. In addition, this process also causes galaxies to form into small groups due to their gravitational attraction. It is thought that those small groups attract more galaxies, repeatedly collide and merge, eventually leading to the larger groups, or clusters, we observe today.
Galaxy clusters are quite young in the universe and are relatively rare. Accordingly, their number density and spatial distribution are strongly influenced by density fluctuations in the early Universe and by the properties of dark matter and dark energy. Thus, using simulations for the overall distribution of galaxy clusters as shown in Figure 1 and comparing them to what we can actually observe, we can learn more about the origin of our universe. Galaxy clusters are one of the main observation targets of the Hyper Suprime-Cam (HSC), an ultra-wide-field primary focus camera developed by the National Astronomical Observatory of Japan (NAOJ) and installed on the Subaru Telescope.
Focusing on inside galaxy clusters, we find gas throughout them at extremely high temperatures of tens of millions of degrees. The atoms are ionized and are in a plasma state. This intracluster medium emits strong X-ray radiation. There is a large pressure gradient between the gas in the center and that near the edges, which creates a force that causes the gas to flow outwards from the center. It is thought that this force is balanced out by the enormous gravitational force of the cluster as a whole (dominated by dark matter), which keeps the shape of the cluster in a state of quasi equilibrium. Consequently, the intracluster medium X-ray intensity distribution is closely related to the distribution of dark matter. Galaxy clusters will be the main target for the upcoming X-Ray Imaging and Spectroscopy Mission (XRISM). To know more about galaxy clusters, we need the development of cosmic magnetohydrodynamics/plasma simulations for comparisons with observations.
The member galaxies in a galaxy cluster move inside of the cluster due to the gravitational forces exerted by the cluster itself, and travel at velocities as much as 1,000 km/s. Such velocities are much higher than the velocities estimated from the total mass of stellar components in the galaxies, so they serve as evidence of the existence of dark matter. In the 1930s, Swiss astronomer Fritz Zwicky estimated the velocities of the member galaxies of the Coma Cluster, and to explain the high numbers, he suggested it was necessary to assume the existence of matter that could not be detected using electromagnetic waves. His work is now considered as the first to suggest at the existence of dark matter using astronomical observations.
It is known that the nature of galaxies depends on the environment around them. For example, galaxies surrounded by numerous other galaxies have less active star formation, and the converse is also true. Within the center of galaxy clusters, where galaxies exist more closely together, we find that most are elliptical and lenticular galaxies, which have less active star formation. There are two main models explaining this characteristic. The first suggests that these galaxies turned out this way due to the environment they were in when first formed, while the second suggests that it came about during the process of the galaxy being formed, then incorporated into a galaxy cluster and interacting with other objects thereafter. However, no clear conclusions have been reached. Harnessing the power of the HSC, we have been able to determine the distribution of galaxies within more galaxy clusters more accurately, and we are successively discovering proto-clusters at great distances of 10 billion light years and more. In addition, the Prime Focus Spectrograph is scheduled to be installed on the Subaru Telescope in the early 2020s, and it is expected to contribute greatly to future large-scale spectroscopic surveys. It is not possible to observe the evolution of a single galaxy cluster, but simulations can be used to look back in time to see how they may have evolved. Observing the formation process in galaxy proto-clusters and comparing that information to galaxy cluster evolution modeled through simulations like that shown in Figure 1 should lead us to a better understanding of how galaxies within galaxy clusters are formed.