Searching for bulk motions in the ICM of massive, merging clusters with Chandra CCD data

Abstract

We search for bulk motions in the intracluster medium (ICM) of massive clusters showing evidence of an ongoing or recent major merger with spatially resolved spectroscopy in {\sl Chandra} CCD data. We identify a sample of 6 merging clusters with $>$150 ks {\sl Chandra} exposure in the redshift range $0.1 < z < 0.3$. By performing X-ray spectral analysis of projected ICM regions selected according to their surface brightness, we obtain the projected redshift maps for all of these clusters. After performing a robust analysis of the statistical and systematic uncertainties in the measured X-ray redshift $z_{\rm X}$, we check whether or not the global $z_{\rm X}$ distribution differs from that expected when the ICM is at rest. We find evidence of significant bulk motions at more than 3$σ$ in A2142 and A115, and less than 2$σ$ in A2034 and A520. Focusing on single regions, we identify significant localized velocity differences in all of the merging clusters. We also perform the same analysis on two relaxed clusters with no signatures of recent mergers, finding no signs of bulk motions, as expected. Our results indicate that deep {\sl Chandra} CCD data enable us to identify the presence of bulk motions at the level of $v_{\rm BM} >$ 1000\ ${\rm km\ s^{-1}}$ in the ICM of massive merging clusters at $0.1<z<0.3$. Although the CCD spectral resolution is not sufficient for a detailed analysis of the ICM dynamics, {\sl Chandra} CCD data constitute a key diagnostic tool complementing X-ray bolometers on board future X-ray missions.