Cold fronts in galaxy clusters I: A case for the large-scale global eigen modes in unmagnetized and weakly magnetized cluster core

Abstract

Galaxy clusters show large-scale azimuthal X-ray surface brightness fluctuations known as cold fronts. Cold fronts are argued to originate due to sloshing driven by sub-halo passage at close proximity to the cluster center. While this causes large-scale perturbations, the physical mechanisms that can sustain spiral density structures are not clear. In this work, we explore whether long wavelength thermal instability is an explanation for cold front formation in a cluster core which is perturbed by sub-halos or AGN activity. Using global linear perturbation analysis, we show that unstable internal gravity waves form large-scale three-dimensional spirals, akin to observed cold fronts. We explore if the presence of magnetic field (along spherical $\hat{\phi}$) may support such structures (by suppressing small scale Kelvin-Helmholtz modes) or disrupt them (by promoting additional thermal instability). We find that latter happens at shorter wavelengths and above characteristic Brunt V\"ais\"al\"a frequency ($>N_{\rm BV}$). Our work implies that large-scale spirals are sustained over a long timescale ($>N^{-1}_{\rm BV}$) even in presence of aligned magnetic fields that is otherwise supportive against mixing at the interface. Secondly, short-wavelength (but relatively longer along the field) unstable compressive modes may form within or in the vicinity of such spirals. The instability is an overstable slow wave, and grows in 2D at timescales $\gtrsim 2-3$ times longer than the spiral growth timescale (via thermal instability). Thus this instability cannot destroy the large scale coherence.