Suzaku Observations of the Centaurus Cluster: Absence of Bulk Motions in the Intracluster Medium

Abstract

Abstract The Centaurus cluster ($z=0.0104$) was observed with the X-ray Imaging Spectrometer (XIS) onboard the Suzaku X-ray satellite in three pointings, one centered on the cluster core and the other two offset by $\pm 8 {{}^{\prime }}$ in declination. To search for possible bulk motions of the intracluster medium, the central energy of a He-like Fe K line (at a rest-frame energy of 6.7 kev) was examined to look for a positional dependence. Over spatial scales of 50 kpc to 140 kpc around the cluster core, the central line energy was found to be constant within a calibration error of 15 eV. The 90% upper limit on the line-of-sight velocity difference is $\vert\Delta v\vert < 1400 \,\mathrm{km} \,\mathrm{s}^{-1}$, giving a tighter constraint than previous measurements. The significant velocity gradients inferred from a previous Chandra study were not detected between two pairs of rectangular regions near the cluster core. These results suggest that the bulk velocity does not largely exceed the thermal velocity of the gas in the central region of the Centaurus cluster. The mean redshift of the intracluster medium was determined to be 0.0097, in agreement with the optical redshift of the cluster within the calibration uncertainty. Implications of the present results for estimating the cluster mass are briefly discussed.