On December 30, 2015, Dr. Paolo Tozzi from Osservatorio Astrofisico di Arcetri in Italy visited NAOC and delivered a lecture about X-rays emitted by the Intracluster Medium (ICM) that pervades the space between galaxies in a cluster and the properties that astronomers can derive based on observing these X-rays. Dr. Tozzi started his talk by showing both optical and X-ray images of Abell 1689 and commenting that although the optical image contains distinct images of galaxies, the X-ray image is diffuse and shows virtually no distinct structure. He exhibited similar optical and X-ray images of the “Bullet Cluster” which he explained gets it name because it is composed of one cluster of galaxies colliding with another cluster of galaxies. He said that this collision is happening at supersonic speeds with respect to the ICM, so the resulting gas dynamics are very similar to those of a supersonic bullet passing through air.  

Dr. Tozzi continued by describing how X-ray observations of clusters can reveal a large amount of information about the clusters and cosmology in general, especially when combined with X-ray spectroscopy. Such measurements can determine the virial mass, gas mass, baryonic fraction and chemical composition of a galaxy cluster. Dr. Tozzi showed some examples of clusters that he and his colleagues have analyzed, as well as related constraints on cosmological parameters derivable from such data. He explained how this line of research complements other observational methods related to cosmology, like cosmic microwave background, supernova surveys and baryon acoustic oscillations, to yield better constraints on cosmological parameters. He also highlighted the advantages of making X-ray observations of galactic clusters with satellites like Chandra and XMM-Newton, and also described the capabilities of a new X-ray satellite that is scheduled to be launched in 2016, ASTRO-H, which will be the first X-ray survey satellite since ROSAT ended its operation in 1999.  

Towards the end of his talk, Dr. Tozzi showed some new results from analysis of motions associated with the ICM of various galactic clusters. He displayed maps of ICM motions that had been measured from blue shifts and red shift of the hot gas, and explained how these maps can reveal the dynamics and internal structure of these clusters of galaxies, helping specialists in this field of study better understand them.