Physics and Astronomy

A study of spiral galaxies seen edge-on has revealed that the "halos" of cosmic rays and magnetic fields above and below the galaxies' disks are much more common than previously thought.

An international team of astronomers, including University of New Mexico Professor Richard Rand, used the Karl G. Jansky Very Large Array (VLA) in Socorro, N.M. to study 35 edge-on spiral galaxies at distances from 11 million to 137 million light-years from Earth.

“This is the most extensive survey of radio halos of galaxies similar to our Milky Way that has ever been done,” said Rand, who is a professor in UNM’s Department of Physics & Astronomy.

Richard Rand

The study took advantage of the ability of the VLA, following completion of a decade-long upgrade project, to detect radio emission much fainter than previously possible.

"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 the project’s leader, Judith Irwin, of Queen's University in Canada.

Spiral galaxies, like our own Milky Way, 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. Learning about the environment above and below such disks has been difficult.

"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 Theresa Wiegert, also of Queen's University, lead author of a paper in the Astronomical Journal reporting the team's findings. The paper provides the first analysis of data from all 35 galaxies in the study.

Rand said a project of this size and scope gives astronomers the ability to look at several different aspects of these galaxies and tackle a variety of scientific goals.

“My interest is in understanding how the cosmic rays and magnetic fields work to push around gas in these halos.  This has bearing on some key issues of galaxy evolution.  Gas may even be ejected from galaxies altogether in this way,” Rand said.

Along with the overall objective of the project, Rand said it also gave UNM undergraduate student, Isaiah Santistevan, the opportunity to work hands-on with astronomical data for this international collaboration.

Isaiah Santistevan

Santistevan is studying astrophysics at UNM and was able to help determine the rate at which these galaxies are forming stars. Santistevan used data from NASA’s WISE (Wide-Field Infrared Survey Explorer) satellite to measure how many new stars are forming every year, how concentrated that formation is, and how it varies within a galaxy, all of which are important for understanding the origin and evolution of the cosmic rays.

“It was very exciting to get him involved and it was a good project for an undergraduate,” said Rand. “He had experience from a previous research project and so he had a good feeling for the image analysis that needed to be done.”

At this point in the project, researchers have been able to see how extensive a "typical" halo is by scaling their images of 30 of the galaxies to the same diameter, then combining them into a single image. The result, said Irwin, 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."

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, as seen with the Very Large Array. At the center is a visible-light image of one of the galaxies, NGC 5775, made using the 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.

The combined image, the scientists said, confirms a prediction of such halos made in 1961.

Along with the report on their findings, the astronomers also are making their first batch of specialized VLA images available to other researchers. In previous publications, the team described the details of their project and its goals. The team has completed a series of VLA observations and their latest paper is based on analysis of their first set of images. They now are analyzing additional datasets, and also will make those additional images available to other scientists when they publish the results of the later analyses.

"The results from this survey will help answer many unsolved questions in galactic evolution and star formation", said Marita Krause of the Max-Planck Institute für Radioastronomie in Bonn, Germany.

The data are available at:

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. This work was supported by the Natural Sciences and Engineering Research Council of Canada.