New insights into mass discrepancy in distant galaxy clusters

For three clusters at moderate redshifts, Miralda-Escude and Babul (1995) reported a discrepancy between the masses estimated from X-ray and from strong gravitational lensing analyses. This discrepancy was suggested to give a possible solution for the so-called baryon fraction problem. We analyze the internal structure of clusters A1689 and A2218. for which galaxy redshifts are available, by applying the method of wavelet analysis, which uses both positions and redshifts of galaxies. The two clusters show the presence of two structures superimposed along the line of sight, which have different mean redshifts and much smaller velocity dispersions than that of the system as a whole. We suggest that these clusters are cases of the on-going merging of two clumps. The cluster alignment, possibly along a large scale structure filament, could explain the high central cluster mass obtained from the lensing analysis. Moreover, the observed X-ray temperature, and thus the derived mass, could be enhanced by collision-heated gas during cluster merging. Our analysis of optical data suggests that the two clusters are less massive than estimated both from lensing and from X-ray analysis. Hence, we are far from lowering the estimate of the baryon fraction in galaxy clusters.