Listening to the extragalactic radio

(Originally published by Max Planck Institute for Radio Astronomy)

October 13, 2015

An international team of astrophysicists, including Troy A. Porter of the Hansen Experimental Physics Laboratory and the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University, has published a new study using radio wave data to delve into the details of "halos" surrounding galaxies.

Spiral galaxy
Composite image of an edge-on spiral galaxy with a radio halo produced by fast-moving particles in the galaxy's magnetic field. 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. (© Jayanne English (U. Manitoba), with support from Judith Irwin and Theresa Wiegert (Queen’s U.) for the CHANG-ES consortium; NRAO/AUI/NSF; NASA/STScI.)

CHANG-ES, the “Continuum Halos in Nearby Galaxies, an EVLA Survey” project, brings together scientists from all over the globe in order to investigate the occurrence and origin of radio halos, to probe the disk-halo interface, and to study in-disk emission as well as their magnetic fields and the cosmic rays illuminating these fields. The goal is to understand connections between radio halos and the host disk and its environment.

A number of scientists from two Max Planck Institutes and Ruhr-University Bochum in Germany are co-authors in a recent study reporting observations of a sample of 35 nearby edge-on galaxies with the compact D-configuration of the Karl G. Jansky Very Large Array (VLA) in a number of spectral windows within two frequency ranges, L-band around 1.5 GHz and C-band around 6 GHz.

A median image constructed from stacking 30 CHANG-ES observations of galaxies at C-band reveals the extent of the galaxy that is otherwise invisible in optical wavelengths.

A study of spiral galaxies seen edge-on has revealed that "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 used the Karl G. Jansky Very Large Array (VLA) to study 35 edge-on spiral galaxies at distances from 11 million to 137 million light-years from Earth. 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," says Judith Irwin, of Queen's University in Canada, leader of the project.

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," says 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.

“We have studied the extended halos of individual galaxies for quite some time”, explains Ralf-Jürgen Dettmar from Ruhr-University in Bochum, Germany. “The CHANG-ES sample will provide an additional statistical access to the important question of galactic feedback.” One of his prime research targets, NGC 5775, was used as template in order to represent the inner star forming region of spiral galaxies (see Fig.).

To see how extensive a "typical" halo is, the astronomers scaled their images of 30 of the galaxies to the same diameter, then another of the authors, Jayanne English, of the University of Manitoba in Canada, combined them into a single image. The result, says Irwin, is "a spectactular image showing that cosmic rays and magnetic fields not only permeate the galaxy disk itself, but extend far above and below the disk."

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", concludes Marita Krause of the Max-Planck Institute für Radioastronomie in Bonn, Germany.

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