Are there places in galaxy clusters where stars are less likely to form? - Strange distribution of galaxies in galaxy clusters existed 7 billion years ago | Observation results

By statistically examining more than 5,000 galaxy clusters existing in the Universe up to 7 billion years ago, captured by the Subaru Telescope’s ultra-wide-field prime focus camera, we identified galaxies that had stopped growing within the clusters. It is clear that the distribution is skewed in the direction of . It indicates the possibility that the mechanism that stops the growth of galaxies works anisotropically inside the galaxy cluster.

Figure 1: An example of the galaxy cluster used in this study. Among the galaxies belonging to the cluster, the blue circles indicate the galaxies that are forming stars, and the orange circles indicate those that have stopped forming stars. Unmarked objects are galaxies and stars unrelated to this cluster. The areas shaded in pink and light blue represent the “aligned” and “perpendicular” directions, respectively, to the long axis of the cluster’s central galaxy. The upper right image is a magnified view of the center of the galaxy cluster. As in this example, the central galaxy basically has a shape close to an ellipse, and the major axis is the direction in which the ellipse extends. Although it is difficult to detect bias in galaxy distribution from observations of individual galaxy clusters, in this study, by analyzing high-quality imaging data of more than 5000 galaxy clusters, we were able to identify growing and non-growing galaxies. We detected a bias in the distribution of galaxies. (Credit: University of Tokyo)


Galaxies, which are groups of hundreds of billions of stars, grow through star formation activity that creates stars using gas as material, but the state of star formation in observed galaxies varies from active to almost stopped. Various up to Investigating under what conditions star formation is promoted or suppressed is important for understanding the growth process of galaxies.

Some galaxies exist alone, while others cluster together. A large group of galaxies consisting of hundreds to thousands of galaxies is called a “cluster of galaxies”. A cluster of galaxies spans 3 million light-years and is filled with high-temperature gas called “cluster gas” with temperatures ranging from tens of millions to hundreds of millions of degrees. Interestingly, many solitary galaxies are star-forming, but many galaxies that belong to clusters have stopped star-forming. This is thought to be due to the unique environment of galaxy clusters, where the galaxy and cluster gas are densely packed. It is known to strip the gas, which is the material of stars, from the interior of the galaxy. As a result, star formation, or growth, in the galaxy is thought to stop.

Much of the research that has focused on galaxy clusters to date has been conducted under the assumption that the properties of galaxies belonging to clusters are isotropic, that is, the properties of galaxies are the same no matter which direction you look from the center of the cluster. It’s been broken However, recent studies have pointed out the possibility that the distribution of galaxies that have stopped growing is skewed in a particular direction within the cluster. Although many galaxy clusters have one giant galaxy (central galaxy) at their center, it seems that galaxies that have stopped growing are more frequent along the long axis of the central galaxy. It is interpreted that this is because the effect of stopping the star formation of galaxies in the cluster is strong in the direction aligned with the central galaxy (major axis direction) and weak in the direction perpendicular to it. Such suggestions are derived from current cosmological studies and observations of a small number of galaxy cluster samples. Therefore, it was unclear whether this bias is universal over a wide range of ages in the universe, or whether it is a general trend seen in all galaxy clusters.

Therefore, a team led by Makoto Ando, ​​a graduate student at the University of Tokyo, captured images using a large-scale survey (Hyper Suprime-Cam Subaru Strategic Framework Program) using the Hyper Suprime-Cam, an ultra-wide-field prime focus camera on the Subaru Telescope. We investigated how the proportion of galaxies that have stopped forming stars changes with the orientation of the central galaxy (Fig. 1). As a result, it was confirmed that the proportion of galaxies that have stopped forming stars is high in the direction along the long axis of the central galaxy, and low in the direction perpendicular to it (Fig. 2). Furthermore, since this bias was detected in galaxy clusters up to about 7 billion years ago, it was found that it is universal regardless of time. The deviation detected this time is small, on the order of a few percent, and could only be detected by statistically analyzing high-quality, large-scale samples of galaxy clusters taken by the Subaru Telescope.

Are there places in galaxy clusters where stars are less likely to form?  - Strange distribution of galaxies in galaxy clusters has existed since 7 billion years ago Fig.2

Figure 2: The bias of galaxies that have stopped growing detected in this study (left) and its image (right). The figure on the left shows the analysis results of the universe about 6 billion years ago, showing the percentage of galaxies that stopped growing (white circles) in each direction from the long axis of the central galaxy. The thick black line is the distribution trend line. The proportion of galaxies that stopped growing in the direction aligned with the long axis of the central galaxy, indicated by pink shading, is higher than in the direction perpendicular to the long axis of the central galaxy, indicated by light blue shading. It’s getting higher. (Credit: University of Tokyo)


So how did this bias come about? In general, it is known that many “massive galaxies” and “galaxies in dense locations” have stopped growing. Therefore, it is possible that (1) there are more massive galaxies along the long axis of the central galaxy, and (2) galaxies are more densely packed along the long axis of the central galaxy. Or (3) galaxies that have stopped growing outside the cluster may be moving into the cluster due to motion along the long axis of the central galaxy. However, when we examine the bias of galaxies detected this time from various angles, we find that (1) and (2) cannot explain the magnitude of the detected bias, and that there is a significant difference in the distribution of galaxies that have stopped growing outside the cluster. We found that there is no bias and the possibility of (3) is also low. Such an explanation does not appear to be sufficient to adequately explain the observed bias.

In fact, a theory that can explain the results of this study well has been proposed in previous research using simulations. It is believed that there is a supermassive black hole at the center of the galaxy. The supermassive black hole in the central galaxy of the cluster emits enough energy to blow out the cluster gas. At this time, since the gas in the direction perpendicular to the long axis of the central galaxy is intensively blown away, the wind pressure exerted on the galaxy by the cluster gas in that direction is relatively weak. As a result, the susceptibility of the growth of galaxies to stop changes depending on the orientation of the central galaxy. The results of this study are basically consistent with this theory. This suggests that the activity of the SMBH in the central galaxy and the interaction between the galaxy and the cluster gas are extremely important in considering the growth of galaxies in the cluster.

Makoto Ando (Graduate School of the University of Tokyo), who led the research, said, “Thanks to the large-scale, high-quality observational data from the Subaru Telescope, we have discovered a new aspect and universality of the mechanisms that stop galaxy growth in galaxy clusters. However, we have not detected the direct evidence of black hole activity or uneven distribution of galaxy cluster gas, which are expected to be clarified by X-ray and radio observations in the future. By clarifying the cause of the bias of the galaxies that have stopped growing, we will be able to understand the growth history of galaxies in galaxy clusters.”

The results of this research were published in the Journal of the Royal Astronomical Society on December 22, 2022 (Ando et al., “Detection of anisotropic satellite quenching in galaxy clusters up to z~1“). This work was supported by the JST Next Generation Researcher Challenging Research Program (JPMJSP2108) and Grants-in-Aid for Scientific Research (22J11975, JP19K03924).

About the Subaru Telescope
The Subaru Telescope is a large optical and infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences, and is supported by the Ministry of Education, Culture, Sports, Science and Technology’s large-scale academic frontier promotion project. Mauna Kea, where the Subaru Telescope is located, is a precious natural environment and an important place in the culture and history of Hawaii. .















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