X-ray observations of a subhalo associated with the NGC 4839 group infalling toward the Coma cluster

We report on Suzaku X-ray observations of the dark subhalo associated with the merging group of NGC 4839 in the Coma cluster. The X-ray image exhibits an elongated tail toward the southwest. The X-ray peak shifts approximately 1' away from the weak-lensing mass center toward the opposite direction of the Coma cluster center. We investigated the temperature, normalization, pressure, and entropy distributions around the subhalo. Excluding the X-ray tail, the temperature beyond the truncation radius is 8-10 keV, which is twice as high as that of the subhalo and the X-ray tail. The pressure is nearly uniform, excluding the southern part of the subhalo at two times of the truncation radius. We computed the gas mass within the truncation radius and the X-ray tail. While the gas fraction within the truncation radius is about five times smaller than that of regular groups, the gas mass in the subhalo and the X-ray tail to weak-lensing mass ratio is consistent with that of regular groups. Assuming an infall velocity of 2000 km s^-1, the ram pressure is 1.4 times greater than the gravitational force per unit area. Assuming the Kelvin-Helmholtz instabilities, the total lost mass is approximately 3 × 10¹¹ M_⊙. If this gas had originally been within the truncation radius, the gas mass fraction of the subhalo would have been comparable with those of regular groups before infalling to the Coma cluster.