Almost all galaxy clusters experience mergers. While 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 (induced by a merger), called “sloshing gas.” Observing a phenomenon similar to sloshing gas in the daily life is easy: when you swirl a wine glass containing some water in it and you will see how the water rotates along with the glass. Discovering how fast the sloshing gas moves in galaxy clusters has profound astronomical meaning and therefore is of great interest to astronomers. A group of researchers from Taiwan and Japan has now measured it by a novel technique enabled by the finest images from the space as well as the ground telescopes available to date; their answer is: “sub-sonic!”

The lead researcher of the study, Dr. Shutaro Ueda from Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) says: “Mergers are the most important factor in the evolution of galaxy clusters. So, measuring the velocity of sloshing gas is crucial for understanding not only the origin of the motion but also the evolution of galaxy clusters. But our knowledge about the velocity of “sloshing gas” is very limited due to the difficulty of measurement. So far, a few results are reported. Thanks to high quality Chandra X-ray and ALMA images, we succeeded in developing a novel technique which can solve the puzzle by combining both images and by calculating the minor perturbations of the gas in a distant galaxy cluster.”

Measuring the moving gas has always been important to astronomers because motion is not only a basic aspect in physics but also an important observable of astronomical objects. The motion of astronomical object can directly tell us what has happened in the “systems.” Therefore, measuring the motion is an important step towards the revealing of where or what the origin is, what the dominating mechanism is, and the nature of the astronomical object itself.

In the case of galaxy clusters, many kinds of features that indicate a presence of motion of intra-cluster medium are found. They are considered to be associated with the evolution of galaxy clusters. It is however hard to measure the motion of intra-cluster medium in galaxy clusters due to the energy resolution of X-ray CCD camera. So, astronomers’ knowledge of the velocity field was limited to only a few samples out of hundreds of galaxy clusters — even through a number of observations have been done in the past two decades. In addition, the measurements were restricted to the line-of-sight velocity of the intra-cluster medium, because only the Doppler effect on emission lines of highly-ionized iron can be used to measure the velocity in X-rays. Recently, a new method is proposed to measure the velocity field by focusing on the perturbation in X-ray images, but the study based on this method is limited so far. Therefore, the measurement of motion of intra-cluster medium is still one of the hottest topics and its velocity is one of the most important physical quantities to understand the nature of galaxy clusters.

The authors of this paper decided to measure the velocity in a luminous X-ray galaxy cluster RX J1347.5-1145 by Chandra and ALMA. Dr. Ueda added: “Of course we need to use Chandra X-ray Observatory and ALMA, because only these two provide us with the high-angular resolution that observing distant galaxy clusters requires. The objects are so tiny on the sky.”

The second author of this paper, Prof. Tetsu Kitayama at Toho University, explains: “We estimated the velocity of the gas in a distant galaxy cluster, for the first time, by combining X-ray and Sunyaev-Zel’dovich effect (SZE) data and by solving the equation of state. In the end, we obtained a direct observational evidence for sub-sonic nature of the sloshing motion in a galaxy cluster 4.8 billion years away from the Earth. This means that the sloshing motion is rather gentle and nearly in pressure equilibrium. We expect that such measurements will become possible for a large number of galaxy clusters once the new ALMA Band 1 receivers, now being constructed under the leadership of ASIAA, are completed.”

Dr. Keiichi Umetsu, a well-known cosmologist and a research fellow of ASIAA, says: “Understanding the physical state of galaxy clusters and their level of equilibrium is essential not only for the physics of galaxy clusters, but also for studies of cosmology, as they are the most massive objects to have formed in the universe. This study offers a new window into the velocity field of the gas in galaxy clusters. From now on, the novel technique developed by the team can add a new dimension of observable for astronomers to explore the nature and evolution of galaxy clusters.”

Reference: “A Cool Core Disturbed: Observational Evidence for Coexistence of Subsonic Sloshing Gas and Stripped Shock-Heated Gas Around the Core of RXJ1347.5-1145,” Shutaro Ueda et al., 2018 Oct. 10, Astrophysical Journal [, preprint:]. The team members are Shutaro Ueda (ASIAA, Taiwan), Tetsu Kitayama (Toho University), Masamune Oguri (The University of Tokyo), Eiichiro Komatsu (Max-Planck Institute for Astrophysics / Kavli Institute for the Physics and Mathematics of the universe, The University of Tokyo), Takuya Akahori (Kagoshima University), Daisuke Iono (National Astronomical Observatory of Japan / SOKENDAI), Takuma Izumi (The University of Tokyo), Ryohei Kawabe (National Astronomical Observatory of Japan / SOKENDAI / The University of Tokyo), Kotaro Kohno (The University of Tokyo), Hiroshi Matsuo (National Astronomical Observatory of Japan / SOKENDAI)

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