Are the Largest-Scale Cosmic Hot Plasmas Heated by Reconnection? K. Makishima (University of Tokyo / RIKEN) Intra-cluster medium (ICM), i.e., the X-ray emitting hot magnetized plasmas confined by the gravity of clusters of galaxies, provide the most dominant form of baryons in the Universe. Previously, the ICM was thought to cool over the Hubble time via radiation, and to accumulate toward the cluster center. The X-ray observations with ASCA (Makishima et al., Publ. Astr. Soc. Japan 53, 401, 2001) have revealed that the ICM is indeed cooler at the center, but is not continuously cooling. Therefore, some unknown mechanism must be heating the ICM. A likely scenario is that the ICM consists of two (hot and cool) phases, the latter confined within numerous magnetic loops anchored to the central dominant galaxy. Then, the kinetic energies associated with the motion of other galaxies are dissipated via reconnection on the ICM. Due to reaction, the galaxies fall gradually toward the potential center. This scenario can explain; (1) the required heating luminosity and thermal stability; (2) the observed temperature gradient and heavy-element distributions of the ICM; (3) relative spatial extents of the galaxies and ICM; and (4) the suggested evidence for ongoing particle acceleration in clusters. If confirmed, this turns out to be the largest-scale magnetic reconnection.