Large scale cluster halos are known only in a few nearby () clusters and in a few higher redshift clusters where the emission interferes with measurement of the Sunyaev-Zeldovich effect. The emission seems correlated with high density clusters which do not show cooling flows. This could indicate that the acceleration of the particles is a byproduct of a cluster-cluster merger which has disrupted any cooling flows and generated a lot of turbulence and/or shock where weakly relativistic particles are accelerated to high enough energies to produce the observed radiation. However, in reality very little is known because the emission is known in so few cases and then is hard to observe above 1.5 GHz or in polarization. Most of the reason for this difficulty is surface brightness sensitivity.
The enhanced VLA should correct this by dramatically improving the continuum sensitivity (especially above 1.5 GHz) and the surface brightness sensitivity using the E configuration. This improvement should allow us to detect halo emission, if present, in many more clusters and lower density regions in poor clusters and the surroundings of rich clusters. It also will allow the study the extent, structure, polarization and spectrum or radio halos, all of which are needed to constrain theories of its origin. This emission seen at higher redshifts may also be the signpost of recent merger activity.