By statistically examining more than 5,000 galaxy clusters that existed 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. is inclined in the direction of . It points to the possibility that the mechanism that halts the growth of galaxies works anisotropically within the galaxy cluster.
Figure 1: An example of the galaxy cluster used in this study. Among the galaxies belonging to the cluster, blue circles indicate galaxies that are forming stars, while orange circles indicate those that have stopped forming stars. Unmarked objects are galaxies and stars unrelated to this cluster. The pink and blue shaded areas represent the “aligned” and “perpendicular” directions, respectively, to the long axis of the cluster’s central galaxy. The top right image is a magnified view of the center of the galaxy cluster. As in this example, the central galaxy is basically shaped like an ellipse, and the major axis is the direction in which the ellipse extends. While it is difficult to detect bias in the distribution of galaxies from observations of individual galaxy clusters, in this study, by analyzing high-quality imaging data from more than 5000 galaxy clusters, we were able to identify growing and non-growing galaxies. . Bias in the distribution of galaxies. (Credit: University of Tokyo)
Galaxies, which are groups of hundreds of billions of stars, grow through star-forming activity that creates stars using gas as material, but the state of star formation in observed galaxies varies from active to nearly stopped. Investigating under which conditions star formation is promoted or suppressed is important for understanding the growth process of galaxies.
Some galaxies exist on their own, while others cluster together. A large group of galaxies consisting of hundreds to thousands of galaxies is called a “cluster of galaxies”. A galaxy cluster 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 forming stars, but many galaxies that belong to clusters have stopped forming. This is thought to be due to the unique environment of galaxy clusters, in which the galaxy and cluster gas are densely packed. It is known to remove gas, which is the material of stars, from within 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 thus far 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 one looks from the cluster center. It was broken. However, recent studies have highlighted 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 a giant galaxy (central galaxy) at their center, it appears that galaxies that have stopped growing are more frequent along the long axis of the central galaxy. This is interpreted to be because the star-stopping effect of the galaxies in the cluster is strong in the direction aligned with the central galaxy (long axis direction) and weak in the direction perpendicular to it. These suggestions derive from current cosmological studies and observations of a small number of galaxy cluster samples. Therefore, it was unclear whether this bias was universal across a wide range of ages in the universe or whether it was a general trend observed across 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 Hyper Suprime-Cam, an ultra-wide – field prime focus camera on the Subaru telescope We studied how the proportion of galaxies that have stopped forming stars changes with the orientation of the central galaxy (Fig. 1). Consequently, 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 has been detected in galaxy clusters up to about 7 billion years ago, it has been found to be 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 large-scale, high-quality samples of galaxy clusters taken by the Subaru Telescope.
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 results of the analysis of the universe about 6 billion years ago, showing the percentage of galaxies that have stopped growing (white circles) in each direction from the long axis of the central galaxy. The thick black line is the trendline of the distribution. The proportion of galaxies that have stopped growing in the direction aligned with the long axis of the central galaxy, indicated by pink shading, is greater than in the direction perpendicular to the long axis of the central galaxy, indicated by light blue shading. . (Credit: University of Tokyo)
So how did this bias come about? In general, many “massive galaxies” and “galaxies in dense places” are known to have stopped growing. Therefore, it is possible that (1) there are more massive galaxies along the long axis of the central galaxy, and (2) the galaxies are more densely clustered along the long axis of the central galaxy. Or (3) galaxies that have stopped growing outside the cluster could enter the cluster due to motion along the long axis of the central galaxy. However, when we examine the polarization of the detected galaxies this time from various angles, we find that (1) and (2) cannot explain the extent of the detected distortion, and that there is a significant difference in the distribution of the galaxies that have stopped growing beyond out of the cluster We found that there is no bias and the possibility of (3) is also low. This explanation does not seem to be sufficient to adequately explain the observed bias.
Indeed, a theory that may well explain the results of this study has been proposed in previous research using simulations. A supermassive black hole is believed to exist at the center of the galaxy. The supermassive black hole in the cluster’s central galaxy emits enough energy to expel the cluster’s gas. At this time, because the gas in the direction perpendicular to the long axis of the central galaxy is pushed away strongly, the wind pressure exerted on the galaxy by the cluster gas in that direction is relatively weak. Consequently, the susceptibility of growing galaxies to stop changes depending on the orientation of the central galaxy. The results of this study are broadly 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 (Tokyo University Graduate School), who led the research, said: “Thanks to the large-scale and high-quality observational data from the Subaru Telescope, we have discovered a new aspect and the universality of the mechanisms that stop growth of galaxies in galaxy clusters.However, we did not detect direct evidence of black hole activity or uneven gas distribution of galaxy clusters, which should be clarified by X-ray and radio observations in the future.Clarifying the cause of growth of galaxies in galaxy clusters. bias of galaxies that have stopped growing, we will be able to understand the history of growth 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, Large-Scale Academic Frontier Promotion Project, of Science and Technology. Mauna Kea, where the Subaru Telescope is located, is a valuable natural environment and an important place in Hawaiian culture and history. .
