Deep Chandra observations of NGC~1404: cluster plasma physics revealed by an infalling early-type galaxy

Yuanyuan Su, Ralph P. Kraft, Elke Roediger, Paul E. J. Nulsen, William R. Forman, Eugene Churazov, Scott W. Randall, Christine Jones, Marie E. Machacek

Published 2016-12-02Version 1

The intracluster medium (ICM), as a magnetized and highly ionized fluid, provides an ideal laboratory to study plasma physics under extreme conditions that cannot yet be achieved on Earth. NGC~1404 is a bright elliptical galaxy that is being gas stripped as it falls through the ICM of the Fornax Cluster. We use the new {\sl Chandra} X-ray observations of NGC 1404 to study ICM microphysics. The interstellar medium (ISM) of NGC 1404 is characterized by a sharp leading edge, 8 kpc from the galaxy center, and a short downstream gaseous tail. Contact discontinuities are resolved on unprecedented spatial scales ($0\farcs5=45$\,pc) due to the combination of the proximity of NGC 1404, the superb spatial resolution of {\sl Chandra}, and the very deep (670 ksec) exposure. At the leading edge, we observe sub-kpc scale eddies generated by Kelvin-Helmholtz instability and put an upper limit of 5\% Spitzer on the isotropic viscosity of the hot cluster plasma. We also observe mixing between the hot cluster gas and the cooler galaxy gas in the downstream stripped tail, which provides further evidence of a low viscosity plasma. The assumed ordered magnetic fields in the ICM ought to be smaller than 5\,$\mu$G to allow KHI to develop. The lack of evident magnetic draping layer just outside the contact edge is consistent with such an upper limit.