Non-thermal Sunyaev-Zel'dovich effect

Question

Exactly what is the non-thermal Sunyaev-Zel'dovich effect? From what I understand from reading several papers by Mark Birkinshaw and Sergio Colafrancesco, I get the rough idea that the non-thermal SZ effect has something to do with it being in the relativistic realm.

Papers by Sergio Colafrancesco:

• Polarization of the Sunyaev-Zel'dovich effect: relativistic imprint of thermal and non-thermal plasma
• The non-thermal Sunyaev-Zel'dovich effect in clusters of galaxies
• Non-thermal phenomena in galaxy clusters: cosmological relevance

Review by Mark Birkinshaw:

The Sunyaev-Zel'dovich effect

To clarify, I'm not referring to the kinematic SZ effect.

There appears to be a distinction by the authors between the SZ effect due to hot thermal electrons and relativistic electrons. I find the nomenclature ambiguous, as temperature is essentially kinetic energy on a smaller scale, so why would relativistic electrons that have more kinetic energy be 'non-thermal'?

Lastly, is the key distinction between them using a relativistic vs. a non-relativistic formulation?

Answer 1

The non-thermal S-Z effect is caused by inverse Compton scattering of the CMB photons from a non-thermal population of electrons - i.e. electrons that have high energies not because they are hot, but because they have been accelerated non-thermally. The usual mechanisms are accelerating by electromagnetic fields and the Lorentz force.

The rest-mass energy of an electron is 0.511 keV. In order to attain "relativistic energies" then the electrons must have kinetic energies similar to this or higher. The average kinetic energy of a particle due to its temperature is just $3k_{B}T/2$. If we equate this to 0.511 keV then the temperature required to have thermal relativistic electrons is $T > 4 \times 10^{6}$ K.

Gas at this temperature does exist in the intracluster medium of massive galaxy clusters. But in addition there are populations of non-thermal electrons that are accelerated to relativistic speeds for instance in the jets and radio-emitting lobes of active galaxies.

The distinction is important when studying the S-Z effect because the energy distributions of thermal and non-thermal populations can be quite different. Non-thermal populations are usually characterised by a power-law, whilst thermal electrons have a Maxwell-Boltzmann distribution. This results in a different inverse Comptonisation spectrum from electrons of the two populations that are often (usually) spatially unresolved. The non-thermal S-Z effect is essentially a contaminant that needs to be accounted for when using the thermal S-Z effect to investigate the structure and parameters of the intracluster gas and using the S-Z effect as a cosmological probe.