The merger in abell 576: A line-of-sight bullet cluster?

Abstract

Using a combination of Chandra and XMM-Newton observations, we confirmed the presence of a significant velocity gradient along the northeast-southwest direction in the intracluster gas of the cluster Abell 576. The results are consistent with a previous ASCA SIS analysis of this cluster. The error-weighted average over the ACIS-S3 and EPIC MOS1 and MOS2 spectrometers for the maximum velocity difference is > 3.3; 10(3) km s(-1) at the 90% confidence level, similar to the velocity limits estimated indirectly for the Bullet Cluster (1E 0657 -56). The probability that the velocity gradient is generated by standard random gain fluctuations with Chandra and XMM-Newton is < 0.1%. The regions of maximum velocity gradient are in CCD zones that have the lowest temporal gain variations. It is unlikely that the velocity gradient is due to Hubble distance differences between projected clusters (probability less than or similar to 0.01%). We mapped the distribution of elemental abundance ratios across the cluster and detected a strong chemical discontinuity using the abundance ratio of silicon to iron, equivalent to a variation from 100% SN Ia iron mass fraction in the west-northwest regions to 32% in the eastern region. The center of the cluster is located at the chemical discontinuity boundary, which is inconsistent with the radially symmetric chemical gradient found in some regular clusters, but consistent with a cluster merging scenario. We predict that the velocity gradient as measured will produce a variation of the cosmic microwave background (CMB) temperature toward the east of the core of the cluster that will be detectable by current and near-future bolometers. The measured velocity gradient opens up the possibility that this cluster is passing through a near line-of-sight merger stage where the cores have recently crossed.