Suzaku X-ray observations of the accreting NGC4839 group of galaxies and a radio relic in the coma cluster

Abstract

Based on Suzaku X-ray observations, we studied the hot gas around the NGC4839 group of galaxies and a radio relic in the outskirts of the Coma cluster. We found a gradual decline in the gas temperature from 5 keV around NGC4839 to 3.6 keV at the radio relic, across which there is a further, steeper drop down to 1.5 keV. This drop as well as the observed surface brightness profile are consistent with a shock with Mach number M = 2.2?0.5 and velocity vs = (1410?110) km s-1. A lower limit of B > 0.33μG was derived based on the magnetic field strength around the relic from the upper limits to inverse Compton X-ray emission. Although this suggests that the non-thermal electrons responsible for the relic are generated by diffusive shock acceleration (DSA), the relation between the measured Mach number and the electron spectrum inferred from radio observations are inconsistent with that expected from the simplest, test-particle theory of DSA. Nevertheless, DSA is still viable if it is initiated by the injection of a pre-existing population of non-thermal electrons. Combined with previous measurements, the temperature profile of Coma in the southwest direction is shallower outside NGC4839, and also slightly shallower in the outermost region. The metal abundance around NGC4839 is confirmed to be higher than that in its vicinity, implying a significant peak in the abundance profile that decreases to 0.2 solar toward the outskirts. We interpret these facts as being due to ram pressure stripping of metal-enriched gas from NGC4839 as it falls into Coma. The relic shock may result from the combined interaction of pre-existing intracluster gas, gas associated with NGC4839, and cooler gas flowing in from the large-scale structure filament in the southwest. © 2013. Astronomical Society of Japan.