The Core Rocks!
Hunting the sloshing gas in the center of massive galaxy clusters
(ASIAA Science Highlight posted on 9th. April, 2020)
Galaxy clusters are the largest gravitationally bound objects in the universe. While it is well known that all galaxy clusters have experienced mergers and collisions through gravitational interactions, how mergers affect the evolution of galaxy clusters is still a mystery. Previous studies conducted by ASIAA astronomers including Dr. Shutaro Ueda have pointed out that “sloshing gas” holds the key to the answers. Now, the team led by Dr. Ueda has systematically evidenced that sloshing gas does exist in all of their clusters sample. The result strongly supports that mergers can always affect the evolution of galaxy clusters and their impacts brought to the entire galaxy clusters can last for a long time.
Dr. Ueda says: "We have analyzed 12 clusters and discovered sloshing gas in all of them. This progress may critically help our understanding towards the cool cores in cluster centers."
"The presence of cool cores is one of the long-standing, well-known problems in astrophysics. They are found in the center of most of the galaxy clusters, the reason we call it "cool core" is because the temperature of the ICM in the center is cooler than that in the surroundings" Dr. Ueda continues: “Indeed, it is very difficult to keep the ICM “relatively-cool” for a long time. If gas temperature becomes low, gas pressure also decreases. Eventually, cool cores must be collapsed quickly. But galaxy clusters have no such collapsing cores, which means that they must have some hidden processes to prevent the collapses. Sloshing gas is one of the clues to trace down this problem.
Explaining how to spot sloshing gas, Dr. Ueda says, "Sloshing gas creates gas density perturbations in the center of galaxy clusters which also attributes a specific pattern of temperature profile. Therefore, spotting these differences is a well-known tool for identifying sloshing gas." The team selected 12 clusters out of CLASH, a world-famous pool for high-mass galaxy clusters images, taken by the Hubble Space Telescope. By analyzing their X-ray images taken by the Chandra X-ray Observatory, the team detected gas density perturbations in all of the selected clusters. They also measured the gas temperature difference. The results are in good agreement with what sloshing gas should look like.
In addition, to test how well their algorithm performs in detecting gas density perturbations and in identifying where those regions locate, the team used synthetic X-ray observations made by a hydrodynamic simulation.
Fig. 1. A simulated X-ray image and its residual image
One of the X-ray surface brightness profiles produced by their numerical simulation (left) and its X-ray residual image after removing its global profile calculated by their novel algorithm (right). Left: Brighter colors (e.g., red and white) and darker colors (e.g., blue) correspond to the bright and faint regions of the X-ray surface brightness, respectively. The cluster center is the center of this image. The white contours show the shape of the dark matter halo. Right: Brighter colors (e.g., red and white) and darker colors (e.g., blue) correspond to the larger positive and negative excess in the X-ray residual image. The white and black areas show the positive and negative excess regions detected by their novel algorithm. The shape of both regions look like spiral, which is a well-known feature of sloshing gas.
Credit: Ueda Shutaro/ASIAA
Fig. 2. One of the observed X-ray images in our cluster sample and its residual image.
One of the X-ray surface brightness profiles produced by their numerical simulation (left) and its X-ray residual image after removing its global profile calculated by their novel algorithm (right). Left: Brighter colors (e.g., red and white) and darker colors (e.g., blue) correspond to the bright and faint regions of the X-ray surface brightness, respectively. The cluster center is the center of this image. The white contours show the shape of the dark matter halo. Right: Brighter colors (e.g., red and white) and darker colors (e.g., blue) correspond to the larger positive and negative excess in the X-ray residual image. The white and black areas show the positive and negative excess regions detected by their novel algorithm. The shape of both regions look like spiral, which is a well-known feature of sloshing gas.
Credit: Ueda Shutaro/ASIAADr. Sandor M. Molnar from ASIAA performed a hydrodynamical simulation of a cluster merger to compare with observations. Dr. Molnar said “Numerical simulations are very powerful tools to understand astrophysics. They can be used to visualize and follow all of physical phenomena that human beings cannot see in their lifetime, namely, for example those, which are happening in colliding galaxy clusters during cosmological time-scale, billions of years. In addition, computer simulations enable us to identify the most important physical mechanisms which can reproduce the observational data and to check for biases in our analysis methods of real data. Thanks to the powerful computing resources at the National Center for High-Performance Computing in Taiwan, we succeeded in calculating physical parameters with very high angular resolution.”
The team is planning a larger number of numerical simulations that may reproduce many types of galaxy clusters. Dr. Ueda explains: “Expanding the number of our cluster sample can reveal how many galaxy clusters host sloshing gas. This information allows us to estimate a lifetime of sloshing gas, which is a key parameter not only to improve our simulations but also to study the property of the ICM.”
Glossary:
Sloshing Gas: Almost all galaxy clusters experience mergers. When a merger takes place, a specific pattern of "spiral" often can be observed in X-ray images. Such a spiral feature is due to the motion of the gas, the so-called "sloshing gas" - induced by a merger. At the beginning of the sloshing gas study, spiral features had been the only recognizable evidence. Later on, in studies like this one, astronomers started using numerical simulations to investigate identifiable evidence of sloshing gas. By now there have been two evidence features that people can look for sloshing gas with.
ICM: the acronym for Intracluster medium, ICM is the superheated plasma that permeates a galaxy cluster. The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters. The ICM is heated to temperatures on the order of 10 to 100 mega kelvins, emitting strong X-ray radiation.
CLASH: Acronym for Cluster Lensing And Supernova survey with Hubble, CLASH is one of three first class Hubble multi-cycle treasury programs designed to tackle large questions unanswerable through normal observations. Observations for CLASH were conducted on the Hubble Space Telescope with images taken in 16 filters, selected to maximize the ability to detect distant galaxies behind each cluster.
Notes:
Paper and Research Team
The paper was published as “Gas Density Perturbations in the Cool Cores of CLASH Galaxy Clusters” in Astrophysical Journal, Volume 892, Number 2
The team members are: Shutaro Ueda, Yuto Ichinohe, Sandor M. Molnar, Keiichi Umetsu, and Tetsu Kitayama
Article author: Dr. Shutaro Ueda
Edited by: Lauren Huang
Reviewed by: Dr. Keiichi Umetsu