Constraints on Intracluster Gas Bulk Motions in Clusters of Galaxies with ASCA

The recent detection of Doppler shifts of X-ray spectral lines in theintracluster gas of the Centaurus and Perseus clusters of galaxies with ASCAhas raised the question of the frequency of such velocity gradients and theirrelation to other cluster properties. To address this and improve the sample ofICM velocity gradient candidates we carried out a systematic search of the ASCAarchive for clusters with suitable observing conditions that allow fullazimuthal mapping of gas velocities near the central regions. Here we presentthe results of the azimuthal velocity distribution of a sample consisting ofthe best" observed clusters for velocity analysis. Our sample encompassesclusters with different physical and morphological characteristics, includingboth well behaved "cooling flow" and dynamically active clusters. The gastemperatures of the clusters in our sample range from 1.4 keV to 6.3 keV. Wefind velocity substructures in ~ 15% of the clusters in our sample on the orderof a few x 1E3 km/s. The significance of the velocity gradients' is greaterthan 99% in Abell 576 and RXJ0419.6+0225, the clusters where velocity gradientsare most significant. The velocity gradients are consistent with transitoryand/or rotational bulk motion. The velocity gradients in these clusters areunlikely to be due to projected temperature anisotropies, ASCA PSF, intrachipgain variations, and flux contamination from other sources. Chandra images ofthese clusters do not show obvious cluster alignments within the field of viewcovered by ASCA. We also noticed a high frequency (~50%) of anisotropictemperature gradients in the core of "cooling flow" clusters, suggesting thepresence of frequent dynamical activity in the core of galaxy clusters.