Chandra Observations of Galaxy Cluster A2218

Abstract

We present results from two observations (combined exposure of ~17 ks) of galaxy cluster A2218 using the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory that were taken on 1999 October 19. Using a Raymond-Smith single-temperature plasma model corrected for galactic absorption, we find a mean cluster temperature of kT=6.9+/-0.5 keV, metallicity of 0.20+/-0.13 (errors are 90% CL), and a rest-frame luminosity in the 2-10 keV energy band of 6.2×10⁴⁴ ergs s^-1 within a 5.1‧ aperture in a Λ cold dark matter cosmology (ΛCDM) with H₀=65 km s^-1 Mpc^-1. The brightness distribution within 4.2‧ of the cluster center is well fitted by a simple spherical beta model with core radius of 66.4" and β=0.705. High-resolution Chandra data of the inner 2' of the cluster show the X-ray brightness centroid displaced ~22" from the dominant cD galaxy and the presence of azimuthally asymmetric temperature variations along the direction of the cluster mass elongation. X-ray and weak lensing mass estimates are in good agreement for the outer parts (r>200 h^-1) of the cluster; however, in the core the observed temperature distribution cannot reconcile the X-ray and strong lensing mass estimates in any model in which the intracluster gas is in thermal hydrostatic equilibrium. Our X-ray data are consistent with a scenario in which A2218 is not in hydrostatic equilibrium because of recent merger activity that has produced significant nonthermal pressure in the core and substructure along the line of sight; each of these phenomena probably contributes to the difference between lensing and X-ray core mass estimates.