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The Forefront of Space Science

Dynamics in a Galaxy Cluster Explored by SUZAKU
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Discovery by SUZAKU

In January 2006, Assoc. Prof. Kiyoshi Hayashida of Osaka University, et al. proposed the long-time observation of galaxy cluster A2256 in the Little Dipper using the Japanese X-ray astronomical satellite SUZAKU. A2256 has two plasma structures (large and small ones) and is a typical "merging galaxy cluster". By using the X-ray CCD onboard SUZAKU, the observation team aimed to measure the Doppler Effect of the X-ray bright lines from the galaxy cluster's plasma. With this observation, we tried to measure the velocity (in the direction of line of sight connecting the earth and the cluster) of the two plasma structures. A similar measurement had been tried before SUZAKU, but the accuracy had not been satisfactory. After fully examining the performance of SUZAKU's detector, we attempted this difficult measurement.

Following the observation in November 2006, Masaaki Nagai, a graduate student of Osaka University at that time, analyzed the data and identified that the velocity difference between the two plasma structures was 1600700km/s. After a while, the analysis for energy calibration of the detector proceeded and we became confident that we could obtain enough accuracy. The author consulted with Hayashida and reanalyzed the data with Shutaro Ueda, a graduate student of Osaka University. We carefully examined the uncertainty to obtain the correct measuring values. As a result we were able to reduce the uncertainty further and concluded that the velocity difference between the two structures, large and small galaxy clusters, was 1500300km/s (Fig. 1). These structures are colliding at a high speed and we anticipate that they will merge several hundreds of millions later (Fig 2).


Figure 1
Figure 1. X-ray spectra including iron line from the small structure of galaxy cluster A2256
With this data, we measured the recession velocity (red shift) of the small structure precisely. Both (a) and (b) show the same date with crosses with error bar. The models (step-like solid line) to reproduce the data differ between (a) and (b). Line (a) is the model that reproduces the data most correctly. In this case, the small structure has less recession velocity than the large one's. Line (b) is the model where the small structure has the same velocity as the large one, i.e., modeled on the assumption that they do not move relatively to each other. The graphs below show ratio of the data and models. The data agrees well with the model in case (a), but gaps are seen between the data and model in (b). Through these data analysis, we concluded that the small structure moves at a speed of about 1500km/s to the large structure.


Figure 2
Figure 2. Conceptual illustration of the merger of the large and small structures when seeing from above, based on our measurement


This is the first time in the world that the velocity difference of galaxy cluster plasmas has been measured using the Doppler Effect. The measurement becomes possible because SUZAKU has the world's highest level of accuracy in CCD sensitivity and energy determination, which was achieved by the steady efforts of a number of researchers engaged in development and calibration of the satellite and detector.

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