Using one of the world’s greatest radio observatories, National Radio Astronomy Observatory’s Very Large Array (VLA), a group of astronomers has found that radio halos around the disks of spiral galaxies are much more common than previously thought.
Composite image of an edge-on spiral galaxy with a radio halo produced by fast-moving particles in the galaxy’s magnetic field. In this image, the large, grey-blue area is a single image formed by combining the radio halos of 30 different galaxies. At the center is a visible-light image of one of the galaxies, NGC 5775, made using the NASA/ESA Hubble Space Telescope. This visible-light image shows only the inner part of the galaxy’s star-forming region, outer portions of which extend horizontally into the area of the radio halo. Image credit: Jayanne English, University of Manitoba / Judith Irwin & Theresa Wiegert, Queen’s University / CHANG-ES Consortium / NRAO / AUI / NSF / NASA / STScI.
The team, directed by Dr Judith Irwin of Queen’s University in Kingston, ON, Canada, observed 35 nearby edge-on spiral galaxies (11 million to 137 million light-years from Earth).
Spiral galaxies – like our own Milky Way Galaxy or the famous Andromeda galaxy – have the vast majority of their stars, gas, and dust in a flat, rotating disk with spiral arms. Most of the light and radio waves seen with telescopes come from objects in that disk.
“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,” Dr Irwin explained.
“Studying these halos with radio telescopes can give us valuable information about a wide range of phenomena, including the rate of star formation within the disk, the winds from exploding stars, and the nature and origin of the galaxies’ magnetic fields,” said team member Dr Theresa Wiegert, also of Queen’s University.
To see how extensive a typical radio halo is, the astronomers scaled their images of 30 of the galaxies to the same diameter and combined them into a single image.
“The result is a spectacular image showing that cosmic rays and magnetic fields not only permeate the galaxy disk itself, but extend far above and below the disk,” Dr Irwin said.
“The combined image confirms a prediction of such halos made in 1961.”
The findings were published online recently in the Astronomical Journal (arXiv.org preprint).
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Theresa Wiegert et al. 2015. CHANG-ES IV: Radio continuum emission of 35 edge-on galaxies observed with the Karl G. Jansky Very Large Array in D-configuration, Data Release 1. Astronomical Journal 150, 3; doi: 10.1088/0004-6256/150/3/81
