Chandra observations of the central cool region of galaxy clusters

We report on the systematic analysis of the central cool gas in 12 cooling flow clusters of galaxies using the Chandra data. These clusters show a temperature decline toward the cluster center. However, contrary to the predictions of cooling flow, the temperatures in the central regions of most of them are remarkably higher and there is no evidence of large amounts of cool gas. On the other hand, we find that radiative cooling surely takes place in the central regions of these clusters. We also find that the strength of the temperature drop in the centers of clusters correlates with the outer temperature, and weakly with the extent of the central cool gas. These correlations imply that richer galaxy clusters have cool cores with larger scales, and the mass of the cool gas is larger than that of elliptical galaxies. Comparing these features with the prospective nature of the heating models, the hierarchical formation model is most preferred.