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A Cluster Catalogue from the ASCA Data

Data from a single source can be used to study properties of the source itself. On the other hand, systematic studies of the same type of sources can provide "source catalogues". These catalogues are crucial to understand the nature of different types of the astronomical objects. Therefore, catalogues are important scientific products as well as scientific papers.

Here we introduce a cluster catalogue derived from ASCA, the 4th Japanese X-ray satellite. All ASCA data are archived at DARTS. ASCA has brought a number of discoveries and achievements in astronomy and astrophysics. One of the achievements is detailed measurement of the properties of the hot gas in clusters of galaxies. A cluster of galaxies is a collection of 100-1000 galaxies. The X-ray emitting hot gas has a much higher mass than the total stellar mass in the galaxies.

Dr. Ota and her colleagues have used DARTS data of 79 clusters, and compiled a catalogue of X-ray clusters. X-ray image (Fig. 1) provides spatial distribution of the hot gas. On the other hand, X-ray spectrum (Fig. 2) provides the temperature and metal abundance.

Fig. 1 (Left): X-ray image of Abell 1689 taken by ROSAT. All figures are provided by Dr. N.Ota.
Fig. 2 (Right): X-ray spectra of Abell 1689 from the ASCA GIS (red) and SIS (blue).

They have made an interesting discovery using the catalogue, that there are two patterns of the spatial distributions of the hot gas, as seen in Fig. 3. Total mass in the clusters is dominated by dark matter. On the dark matter, by definition, only gravitational force works. Therefore, the gravity should be a dominant factor for the dynamical evolution of the clusters. If so, all the clusters should have evolved along with a single path in almost the same way as each other (=self-similar evolution). This simple model cannot explain Ota's discovery. Their finding clearly indicates that some other force(s) is/are working significantly on the cluster evolution. May exist the dark matter cores in some clusters? May X-ray radiation cooling and plasma concentration toward the cluster center have taken place? A complete solution has not yet been given. We will hopefully find the answer soon from recent observations and numerical simulations of the cosmic structure formations.

Fig. 3 (right): Core radius distribution of the 79 clusters. The core radius is a typical size of the cluster core from a model fitting of the hot gas distribution. There are two peaks at log[R/Mpc]=-0.3 and -0.7. These correspond to 50 kpc and 200 kpc.

A detailed result can be found below.

Title of paper	Authors	Object Type	number of objects
A uniform X-ray analysis of 79 distant galaxy clusters with ROSAT and ASCA	Naomi Ota and Kazuhisa Mitsuda	cluster of galaxies	79

Takayuki Tamura (ISAS/JAXA)

January 2008