Radio Halo

I thought at first that the astronomical term "radio halo" might not be literal. It turns out to be quite apt.  It is a large area of radio emission found in a cluster of galaxies. Astronomers have discovered nearly 300 galaxy clusters and groups,  But few of these have radio halos.  They are very rare, found only in a fraction of those massive clusters. The cause is unknown and hotly debated. Because that's what we do in science.  One theory suggests that they're formed by the turbulence created by colliding clusters of galaxies. Another theory suggests they are created in the complex mergers of galaxy clusters. A third suggests they are a byproduct of black holes or jetting plasma.More here.

Radio halos are enormous. It might be 40 kiloparsecs in length. [For reference One parsec is approximately 3.262 light-years.  A kiloparsec is one thousand parsecs.] They are found at the center of galaxy clusters. Nearly all radio galaxies fall into two categories.

There is a spatial correlation between the brightness of a radio halo and the hot gas regions of a galaxy cluster. This is the opposite of radio relics which though similar, are found at the edge of galaxy clusters. I imagine them like bubbles of radio wave intensity resulting from a cosmic event. As you might expect, their frequencies are usually clustered high in the GHz range.  But not always...

There are exceptions. In 2008 a mixed group of scientists cooperating (Istituto di Radioastronomia, Harvard-Smithsonian Center for Astrophysics, Naval Research Laboratory, Dipartimento di Astronomia, Università di Bologna and The National Radio Astronomy Observatory) reported the following:

"...a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz...The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that manya radio haloes in the Universe should emit mainly at low frequencies."

It's a low frequency radio halo. They continue to study the phenomena with LOFAR and LWA radio telescopes.  More here.