A Deep X-Ray Look At Abell 2142 --- Viscosity Constraints From Kelvin-Helmholtz Eddies, A Displaced Cool Peak That Makes A Warm Core, And A Possible Plasma Depletion Layer

TL;DR

This study uses deep Chandra X-ray observations of galaxy cluster A2142 to analyze cold front structures, constrain plasma viscosity, and identify features like a warm core and possible plasma depletion layer.

Contribution

It provides detailed analysis of cold front Kelvin-Helmholtz eddies, constrains plasma viscosity to be at most 1/5 of Spitzer value, and reports a displaced cool core and potential plasma depletion layer.

Findings

Kelvin-Helmholtz eddies observed at the southern cold front.

Effective viscosity constrained to ≤ 1/5 of Spitzer isotropic viscosity.

Identification of a displaced cool core and a possible plasma depletion layer.

Abstract

We analyzed 200 ks of Chandra ACIS observations of the merging galaxy cluster A2142 to examine its prominent cold fronts in detail. We find that the southern cold front exhibits well-developed Kelvin-Helmholtz (KH) eddies seen in the sky plane. Comparing their wavelength and amplitude with those in hydrodynamic simulations of cold fronts in viscous gas, and estimating the gas tangential velocity from centripetal acceleration, we constrain the effective viscosity to be at most 1/5 of Spitzer isotropic viscosity, but consistent with full Braginskii anisotropic viscosity for magnetized plasma. While the northwestern front does not show obvious eddies, its shape and the structure of its brightness profile suggest KH eddies seen in projection. The southern cold front continues in a spiral to the center of the cluster, ending with another cold front only 12 kpc from the gas density peak. The cool peak itself is displaced ~30 kpc from the BCG (the biggest such offset among centrally-peaked clusters), while the X-ray emission on a larger scale is still centered on the BCG, indicating that the BCG is at the center of the gravitational potential and the cool gas is sloshing in it. The specific entropy index of the gas in the peak ($K\approx49$ keV cm$^2$) makes A2142 a rare "warm core"; apparently the large displacement of the cool peak by sloshing is the reason. Finally, we find a subtle narrow, straight channel with a 10% drop in X-ray brightness, aligned with the southern cold front --- possibly a plasma depletion layer in projection.