Search for gas bulk motions in eight nearby clusters of galaxies with Suzaku

TL;DR

This study used Suzaku X-ray spectra to measure gas motions in eight galaxy clusters, finding mostly small velocities but hints of significant bulk flows in some, highlighting the importance of nonthermal pressure in cluster mass estimates.

Contribution

It presents the first systematic search for intracluster gas bulk motions in multiple clusters using Suzaku, providing upper limits and potential detections relevant for cluster dynamics.

Findings

Gas bulk velocities in cluster cores do not exceed the sound speed.

Significant velocity offset detected in Cen45 suggests ongoing merger activity.

Evidence of large bulk motions in some clusters indicates nonthermal pressure support.

Abstract

To search for bulk motions of the intracluster medium, we analyzed the X-ray spectra taken with the Suzaku satellite and measured the Doppler shift of Fe-K line emission from eight nearby clusters of galaxies with various X-ray morphologies. In the cores of the Centaurus and Perseus clusters, the gas bulk velocity does not exceed the sound velocity, which confirms the results of previous research. For the Cen45 subcluster, we found that the radial velocity relative to the Centaurus core, <780 km s^-1, is significantly smaller than that reported in the optical band at the 3.9 sigma level, which suggests an offset between the gas and galaxy distributions along the line of sight due to the subcluster merger. In A2199, A2142, A3667, and A133, no significant bulk motion was detected, indicating an upper limit on the radial velocity of 3000-4000 km s^-1. A sign of large bulk velocity in excess of the instrumental calibration uncertainty was found near the center of cool-core cluster A2029 and in the subcluster of the merging cluster A2255, suggesting that the nonthermal pressure support is not negligible in estimating the total gravitational mass of not only merging clusters but also relaxed clusters as predicted by numerical simulations. To improve the significance of the detection, however, a further examination by follow-up observations is required. The present study provides a pilot survey prior to the future high-resolution spectroscopy with ASTRO-H, which is expected to play a critical role in revealing the dynamical evolutions of clusters.