Measuring the non-thermal pressure in early type galaxy atmospheres: A comparison of X-ray and optical potential profiles in M87 and NGC1399

TL;DR

This study compares X-ray and optical data to estimate non-thermal pressure contributions in galaxy atmospheres, providing upper limits on magnetic fields and cosmic ray energy densities, supporting models of AGN-regulated cool cores.

Contribution

It offers the first constraints on non-thermal pressure components in galaxy atmospheres using combined X-ray and optical potential profiles.

Findings

Non-thermal pressure contributes up to ~10% of gas thermal pressure.

Magnetic fields are limited to ~10-20 microGauss at 1'-2' from galaxy centers.

Relativistic protons account for less than ~10-20% of energy density.

Abstract

We compare the gravitational potential profiles of the elliptical galaxies NGC 4486 (M87) and NGC 1399 (the central galaxy in the Fornax cluster) derived from X-ray and optical data. This comparison suggests that the combined contribution of cosmic rays, magnetic fields and micro-turbulence to the pressure is ~10% of the gas thermal pressure in the cores of NGC 1399 and M87, although the uncertainties in our model assumptions (e.g., spherical symmetry) are sufficiently large that the contribution could be consistent with zero. In the absence of any other form of non-thermal pressure support, these upper bounds translate into upper limits on the magnetic field of ~10-20 muG at a distance of 1'-2' from the centers of NGC1399 and M87. We show that these results are consistent with the current paradigm of cool cluster cores, based on the assumption that AGN activity regulates the thermal state of the gas by injecting energy into the intra-cluster medium. The limit of ~10-20% on the energy density in the form of relativistic protons applies not only to the current state of the gas, but essentially to the entire history of the intra-cluster medium, provided that cosmic ray protons evolve adiabatically and that their spatial diffusion is suppressed.