Origin of cool cores, cold fronts, and spiral structures in cool core clusters of galaxies

Abstract

Abstract We consider a situation in which a brightest cluster galaxy (BCG) moves in ambient hot gas in the central region of a cool core cluster of galaxies, following the study by Inoue (2014, PASJ, 66, 60). In the rest frame of the BCG, the hot gas is supposed to flow toward the BCG in parallel from a sufficiently large distance. Then, it is expected that only the gas flowing with an impact parameter less than a critical value is trapped by the gravitation field of the BCG because of the efficient radiative cooling, getting a cooling flow, and that the remaining outer gas can get over the potential well. In such a circumstance, we can draw the following picture: A boundary layer between the out-flowing gas and the trapped gas arises around the stagnation point at the back side of the BCG. Since the boundary temperature is so low as to be X-ray dim, the boundary could be observed as the cold front in X-rays. The trapped gas stagnates on the inner side of the boundary and starts in-falling toward the BCG. Since the wandering motion of the BCG is likely to have a rotational component, the Coriolis force induces a rotational motion in the in-falling flow from the stagnation place to the BCG, forming a spiral structure around the BCG. The spiraling flow turns the BCG on the up stream side of the main flow from the far outside, and another boundary layer arises that has contact discontinuity with the main hot gas flow. These pictures reproduce the observed features such as cool cores, cold fronts, and spiral structures well. It can also be resolved how the cooling flow is suppressed from what the cooling flow hypothesis predicts, without any heating mechanism.