After studying 35 spiral galaxies, an international team of astronomers has discovered that these objects are commonly surrounded by a halo of magnetic fields and cosmic rays.
The research, published in the Astronomical Journal, was conducted using the Karl G. Jansky Very Large Array (VLA), the radio astronomy observatory made famous in popular culture by the movie Contact. As the name suggests, it's pretty big, comprising 27 radio antennas in a Y-shaped array, each measuring 25 meters (82 feet) in diameter. The signal received by the antennas is combined together which allows for higher precision. The observatory has recently been upgraded and the resulting enhancement in sensitivity made this present research possible.
The team selected 35 edge-on spirals, or those that face us side-on (as in the image above), at distances between 11 million and 137 million light-years from Earth. The edge-on types were selected so that the telescope could detect the surrounding halo emission without having to distinguish it from other sources – if astronomers were to observe a spiral galaxy face on, telescopes would pick up a bounty of radio signals coming from objects within the disk, like stars. When charged particles in cosmic rays interact with magnetic fields, electromagnetic radiation is emitted; in this case radio waves.
Galaxy halos (not to be confused with dark matter halos) were first discovered in 1959 as an extended radio emission in the Coma Cluster, one of the largest nearby clusters of galaxies, with over 1,000 members. It was predicted that many galaxies could have such emission halos, but there was a significant lack of evidence.
"We knew before that some halos existed, but, using the full power of the upgraded VLA and the full power of some advanced image-processing techniques, we found that these halos are much more common among spiral galaxies than we had realized," said leader of the project Judith Irwin, of Queen's University in Canada, in a statement.
To better visualize the typical halo, 30 of the 35 galaxies studied by the team were scaled and combined together in a single image. The image shows the extent of the radio halo, highlighting how magnetic fields and cosmic rays stretch far beyond the galactic disk.
The team believes that by understanding radio halos, they’ll be able to probe phenomena like the particle winds generated by supernovae and the origin of galaxy-wide magnetic fields. The team also expects to be able to estimate the star-formation rates of these galaxies based on the radio halo extent, a task that would be very difficult using traditional techniques.