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

TL;DR

This study uses deep Chandra X-ray observations of NGC 1404 to investigate plasma physics in the intracluster medium, revealing details about viscosity, magnetic fields, and gas mixing at unprecedented spatial resolution.

Contribution

It provides new insights into ICM microphysics by resolving contact discontinuities and eddies, and constrains plasma viscosity and magnetic field strength in a galaxy cluster environment.

Findings

Sub-kpc scale eddies generated by Kelvin-Helmholtz instability observed.

Upper limit of 5% Spitzer viscosity for hot cluster plasma.

Evidence of mixing between hot ICM and cooler galaxy gas.

Abstract

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.