Shock fronts, electron-ion equilibration and intracluster medium transport processes in the merging cluster Abell 2146

Abstract

We present a new 400-ks Chandra X-ray observation of the merging galaxy cluster Abell 2146. This deep observation reveals detailed structure associated with the major merger event including the Mach number M = 2.3 ± 0.2 bow shock ahead of the dense, ram pressure stripped subcluster core and the first known example of an upstream shock in the intracluster medium (ICM) (M = 1.6 ± 0.1). By measuring the electron temperature profile behind each shock front, we determine the time-scale for the electron population to thermally equilibrate with the shock-heated ions. We find that the temperature profile behind the bow shock is consistent with the time-scale for Coulomb collisional equilibration and the post-shock temperature is lower than expected for instant shock heating of the electrons. Although like the Bullet cluster the electron temperatures behind the upstream shock front are hotter than expected, favouring the instant heating model, the uncertainty on the temperature values is greater here and there is significant substructure complicating the interpretation. We also measured the width of each shock front and the contact discontinuity on the leading edge of the subcluster core to investigate the suppression of transport processes in the ICM. The upstream shock is ∼440 kpc in length but appears remarkably narrow over this distance with a best-fitting width of only 6 +5 −3 kpc compared with the mean free path of 23 ± 5 kpc. The leading edge of the subcluster core is also narrow with an upper limit on the width of only 2 kpc separating the cool, multiphase gas at 0.5-2 keV from the shock-heated surrounding ICM at ∼6 keV. The strong suppression of diffusion and conduction across this edge suggests a magnetic draping layer may have formed around the subcluster core. The deep Chandra observation has also revealed a cool, dense plume of material extending ∼170 kpc perpendicular to the merger axis, which is likely to be the disrupted remnant of the primary cluster core. This asymmetry in the cluster morphology indicates the merger has a non-zero impact parameter. We suggest that this also explains why the south-western edge of the subcluster core is narrow and stable over ∼150 kpc in length, but the north-eastern edge is broad and being stripped of material.

Electron Heating in Low Mach Number Perpendicular Shocks. II. Dependence on the Pre-shock Conditions

“…1 While Coulomb collisions will eventually drive electrons and protons to equal temperatures, the collisional equilibration timescale (Spitzer 1962) for typical conditions in the intracluster medium (ICM) is as long as 10 8 − 10 9 yrs. In fact, X-ray observations by Russell et al (2012) have shown that the electron temperature just behind a merger shock in Abell 2146 is lower than the mean gas temperature expected from the Rankine-Hugoniot jump conditions, and thus lower than the proton temperature. On the other hand, Markevitch (2006) found that the temperatures across the shock in 1E 0657-56 (the so-called "Bullet cluster") are consistent with instant shock-heating of the electrons.…”

Electron Heating in Low-Mach-number Perpendicular Shocks. I. Heating Mechanism

“…While Coulomb collisions will eventually drive electrons and protons to equal temperatures, the collisional equilibration timescale (Spitzer 1962) for typical ICM conditions is as long as 10 8 − 10 9 yrs. In fact, X-ray observations by Russell et al (2012) have shown that the electron temperature just behind a merger shock in Abell 2146 is lower than the mean gas temperature expected from the Rankine-Hugoniot jump conditions, and thus lower than the proton temperature. As a separate evidence, Akamatsu et al (2017) has compiled a list of merger shocks, estimating their Mach number from both X-ray (M s,X−ray ) and radio observations (M s,radio ), and noticed a slight bias of M s,radio M s,X−ray .…”

Complex Diffuse Radio Emission in the Merging Planck Esz Cluster A3411

“…Fainter X-ray emission is found surrounding the subcluster core and this emission seems to be part of a second, larger subcluster that likely extends beyond the northern boundary of the Chandra image. There is no clear surface brightness peak corresponding to the primary cluster core, which suggests the primary cluster has been disrupted by the collision with the subcluster, as has been the case for A2146 (Russell et al 2012). The ICM temperature was fit using XSPEC with an absorbed singletemperature APEC model, which describes emission from collisionally-ionized diffuse plasma.…”