The Self-limiting Nature of Jet-modulated Thermal Conduction in Cool Core Clusters

Abstract Conduction as a mechanism for explaining the disrupted cooling-flow in galaxy clusters has been mostly discounted, as the process is inefficient at transporting heat all the way from the cluster into the core. However, thermal conduction can be strongly enhanced when materials of significantly different temperature are brought into proximity, and thus into close thermal contact. Jets of active galactic nuclei (AGNs) may act as heat pumps by bringing low-entropy gas from the cluster core into thermal contact with the hot outer atmosphere of the cluster, significantly increasing the feedback efficiency of AGNs. We test this hypothesis by running a suite of 3D magnetohydrodynamic simulations of AGNs jets in a Perseus-like cluster, including anisotropic conduction. We find that the heat pump efficiency η can reach up to 50% of the maximum possible efficiency <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>η</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> </mml:math> if conduction operates near the Spitzer–Braginskii limit, while <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>η</mml:mi> <mml:mo>≈</mml:mo> <mml:mspace width="0.25em"/> <mml:msub> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">sp</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>η</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> </mml:math> if conduction along the field lines is substantially suppressed below the Spitzer–Braginskii value by a factor f sp by kinetic effects, as recently suggested. We further find that jet-induced thermal conduction is self-limiting. Magnetic draping during the uplift results in a magnetic field orientation close to perpendicular to the induced temperature gradients, significantly reducing conduction along the ideal conductive pathways. Thus, for conservative assumptions about thermal conduction suppression by f sp ≲ 0.1, the heat pump effect leads to only marginal heat transfer and, correspondingly, to immaterial changes in the overall thermal evolution of cool core clusters beyond the isolated effects of conduction and jet-induced heating alone.