Searching for Bulk Motions in the Intracluster Medium 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 Chandra CCD data. We identify a sample of six merging clusters with >150 ks 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 X , we check whether or not the global z X distribution differs from that expected when the ICM is at rest. We find evidence of significant bulk motions at more than 3σ in A2142 and A115, and less than 2σ in A2034 and A520. Focusing on single regions, we identify significant localized velocity differences in all of the merger 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 Chandra CCD data enable us to identify the presence of bulk motions at the level of v BM >1000 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, Chandra CCD data constitute a key diagnostic tool complementing X-ray bolometers on board future X-ray missions.

The Unrelaxed Dynamical Structure of the Galaxy Cluster Abell 85

“…Liu et al (2015Liu et al ( , 2016 outlined a simple and effective technique to measure the projected X-ray redshifts in different ICM regions and assess the statistical and systematic errors of these redshifts.…”

Reconstruction of Radio Relics and X-ray Tails in an Off-axis Cluster Merger: Hydrodynamical Simulations of A115

“…They also demonstrate the robustness of this result by including in their analysis the member galaxies within 250 kpc radius from each BCG, although the significance is somewhat lower (838 ± 549 km s −1 ). Using X-ray spectra, Liu et al (2016) report an extremely large velocity difference (∼4600 ± 1100 km s −1 ) between A115N and A115S. Although we think that more studies are needed to settle the A115 LOS velocity issue, here we discuss how the results change when we assume a non-zero viewing angle.…”

Testing the rotation versus merger scenario in the galaxy cluster Abell 2107

“…This value corresponds to the emission-weighted average redshift of all the ICM components observed along the line of sight in the selected region. The measurement of ICM redshift and the assessment of its uncertainty have been described and applied in Liu et al (2015Liu et al ( , 2016 and Liu et al (2018). From the difference of the two redshifts, we obtain the velocity difference between the two regions as ∆v =…”

“…The uncertainty on the redshift σ z strongly depends on the strength of the signal, on the modelization of the thermal structure of the ICM, and on calibration issues, and typically is found in the range 0.002 < σ z < 0.01 for bright, nearby clusters and groups observed with Chandra. We refer the reader to Yu et al (2011) for a discussion on the accuracy on the global ICM redshift and to Liu et al (2015Liu et al ( , 2016 for the accuracy in spatially resolved analysis of the ICM. Therefore, with a typical uncertainty on ICM velocity in nearby clusters in the range c × σ z /(1 + z) ∼ 500 − 2000 km/s, the search of bulk motions provided positive results only in a few clusters with highly disturbed dynamical status (see Parekh et al 2015;Liu et al 2016, and references therein), including the remarkable case of the Bullet Cluster, where supersonic bulk motion has been identified along the line of sight and perpendicularly to the merger axis (Liu et al 2015).…”