Novel Technique turns Chandra/ ALMA images into a "Speedometer" for Sloshing Gas!

(ASIAA, the Academia Sinica Institute of Astronomy and Astrophysics, released the news on October 18th., 2018)

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 image of the X-ray Surface brightness of RXJ 1347.5-1145 taken with Chandra. This image shows the electron density distribution of intra-cluster medium. The electron density in the central region (red and brighter color in this image) is much higher than that in the outskirts of this cluster (blue and darker color). Note that the color is artificial, so it indicates only a level of electron density. The green cross indicates the center of this cluster. The image is slightly axis-symmetric and elongated toward the left-bottom direction. This elongation is associated with an infalling subcluster. The feature of the subcluster appears in Figure 2 clearly. Image Credit: Chandra /Ueda et al.

Fig. 2 : The residual image of Figure 1 after applying our novel technique to find substructures in galaxy clusters. A spiral-like feature appears in the central region of this cluster, which is evidence of "sloshing gas". The sloshing gas is considered to be moving around the center of this cluster. In addition, a structure in left-bottom side shown in red color exists. This is evidence of a merger in this cluster. Image Credit: Chandra /Ueda et al.

Fig. 3 : The image of the Sunyaev-Zel'dovich effect of RXJ 1347.5-1145 taken with ALMA. The signal of this effect is shown as blue and darker color in this image. Note that the color is artificial, so it indicates only a level of the signal of this effect. The entire morphology is similar to the X-ray surface brightness (Figure 1). The position of the center of this cluster is shown as the green cross. Image Credit: ALMA/Ueda et al.

Fig. 4 : The residual image of Figure 3 after applying the same technique as Figure 2. Interestingly, no spiral pattern is found in the central region of this cluster. This difference between the residual images of X-ray and the Sunyaev-Zel'dovich effect tells us important information about a velocity of sloshing gas. On the other hand, the feature in left-bottom side is evidence of a merger in this cluster as shown in Figure 2. Image Credit: ALMA/Ueda et al.

Movie 1:Created using Figure 1 and Figure 2, the movie shows what's happened in the galaxy cluster RXJ 1347.5-1145. i.e., before and after applying the novel technique to the X-ray surface brightness of this cluster.

Image Credit: Chandra /Ueda

Youtube link:

GIF link:

video composed by ASIAA/Lauren Huang

Movie 2: Created using Figure 3 and Figure 4, the movie shows what's happened in the galaxy cluster RXJ 1347.5-1145, i.e., before and after applying the novel technique to the data of Sunyaev-Zel'dovich effect.

Image Credit: ALMA/ Ueda

Youtube link:

GIF link:

video composed by ASIAA/Lauren Huang

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.”

Related terms explained

Galaxy cluster

Galaxy clusters are the largest gravitationally bound objects in the Universe. Hundreds to thousands of galaxies are bounded in the gravity of galaxy clusters, which is mainly formed by dark matter with typical mass range of 1014 - 1015 solar mass (i.e., 1047 - 1048 gram). In 1970s, X-ray astronomers discovered that most of normal matters in galaxy clusters exist as optically-thin, hot (107 - 108 Kelvin) plasma so-called intra-cluster medium, whose total mass is a factor of several larger than that integrated with all of galaxies. Galaxy clusters are therefore important systems for understanding the evolution of matter in the dark matter-formed structure in the Universe.


Chandra X-ray Observatory is one of the working X-ray astronomy satellites launched on July 23, 1999. It was developed by NASA as NASA's flagship mission. Chandra has the best angular resolution in X-ray observatories, which is the most important capability to get high quality images of X-ray emitting-astronomical objects.


The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical radio interferometer in the Atacama Desert of Chile, operated by the international partnership among Europe, the United States, Canada, Japan, South Korea, Taiwan, and Chile. The imaging capability and sensitivity of ALMA enable us, for the first time, to observe high-resolution image of the Sunyaev-Zel'dovich effect, which is comparable to X-ray image taken by Chandra. ASIAA has been leading to conduct the development of the ALMA Band 1 receiver, which will provide us with high-resolution, high-sensitivity observations of the Sunyaev-Zel'dovich effect. So, astronomers are waiting for its completion and starting full operation. and

The Sunyaev-Zel'dovich effect

When photons of the cosmic microwave background radiation go through galaxy clusters, they interact with high energy electrons in intra-cluster medium via inverse Compton scattering. The energy of the photons is then changed and a difference between the original and changed ones appears in the spectrum of cosmic microwave background radiation. This effect is sensitive to pressure of intra-cluster medium, so it is a good tracer of very hot intra-cluster medium with temperature of over 100 million Kelvin. Over from 1960s to 1970s, Dr. Rashid Sunyaev and Dr. Yakov Zel'dovich predicted this effect. Later, this effect was indeed found, so it is called the Sunyaev-Zel'dovich effect in praise of their prediction.

RXJ 1347.5-1145 :

RXJ 1347.5-1145 is one of the most massive galaxy clusters in the Universe. This cluster is also known as a merging cluster, which means that this cluster is now growing drastically. Astronomers therefore consider that this cluster is an important target for revealing the evolution of galaxy clusters as well as high energy phenomena of intra-cluster medium. The project team of this paper has succeeded in observing the Sunyaev-Zel'dovich effect of this cluster taken by ALMA for the first time (PI: Prof. Tetsu Kiayama, who is the second author of this paper).

Additional information:

This research was presented in a paper "A Cool Core Disturbed: Observational Evidence for Coexistence of Subsonic Sloshing Gas and Stripped Shock-heated Gas around the Core of RXJ1347.5-1145" by Ueda et al. published in the Astrophysical Journal on 2018 October 10.

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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