Global Magneto-Ionic Medium Survey

The NRC Herzberg Institute of Astrophysics (NRC-HIA) is leading a project to survey the polarized emission over the entire sky, over the widest wavelength range ever attempted from 16 cm up to 1 metre (300 MHz to 1.8 GHz). In this global venture, 10 institutions around the world are participating to study the magneto-ionic medium of our Galaxy - the medium composed of magnetic fields and electrons.

Participating Institutions:

• University of Calgary, Canada
• Australia Telescope National Facility, Australia
• University of Sydney, Australia
• University of Manchester, UK
• University of Newcastle upon Tyne, UK
• Istituto di Radioastronomia, Italy
• Max-Planck Institut fuer Radioastronomie, Germany
• National Radio Astronomy Observatory, USA
• National Astronomical Observatories Beijing, China
• University of Tasmania, Australia

Magnetism plays an important role in the Universe: the Earth, the Sun, all other stars, and all galaxies have magnetic fields. Our own galaxy, the Milky Way, has a magnetic field which roughly follows its spiral arms. The Milky Way field is one million times weaker than the Earth's field but stores so much energy that it plays a significant role in the birth and death of stars. Ironically, we know more about magnetic fields in galaxies outside our own. The Global Magneto-Ionic Medium Survey (GMIMS) will provide astronomers important data to learn more about the origin of the magnetic field in the Milky Way and how it influences the interstellar medium from star formation, to the grand design of our Galaxy.

An important tool for studies of magnetic fields is mapping of radio synchrotron emission. Radio signals generated in the Milky Way bear the imprint of the magnetic fields, seen as a preferred orientation in the received signal. The polarization direction of synchrotron emission is perpendicular to the magnetic field. So, in principle, by mapping this emission we can get a picture of the magnetic field. However, another effect, the Faraday rotation, destroys this information by rotating the polarization direction. The picture below shows the fraction of the synchrotron emission that is polarized. Without Faraday rotation, the dark region in the centre of the map would be as bright as the two regions on the top and at the left-hand side of the map. This depolarization can be overcome by observing at many different wavelengths.

For cosmologists, the Milky Way is just an unwanted foreground, a dirty windshield that they must peer through to observe the so-called Cosmic Microwave Background (CMB) emission - the relict radiation from the big bang. A precise knowledge of the Galactic foreground is required to discern the finer details of the CMB. GMIMS will provide cosmologists with the data required to accurately subtract the Galactic synchrotron emission.

And in the not so distant future, the Square Kilometre Array (SKA), the biggest and most sensitive radio telescope ever conceived, will allow astronomers the deepest view into the universe and will give us the most detailed maps of galaxies. Investigating cosmic magnetism is a major science goal for the SKA. Over the next few years, GMIMS will give us a glimpse of what the SKA will reveal in the future. Our experiences with GMIMS will help us design future telescopes and exploit them to the full.

Only a tiny fraction of the wavelength range that GMIMS aims to cover is actually protected for radio astronomy. The rest is shared with radio signals from cell phones, broadcast stations, satellite transmissions, and so forth. Fortunately, the rapid development of digital signal processors has made it possible to observe "through" this by selecting regions that are still reasonably empty of radio interference.

Because of the wide wavelength range, different telescopes with different receivers need to be used, located in the Northern and Southern hemispheres. The 26-m Telescope of the NRC Herzberg Institute of Astrophysics (picture below), located in Penticton, plays an important role in GMIMS. Over the past three years, the receiving system of the 26-m Telescope has been upgraded for GMIMS. Along with a wide-band receiver, partly designed by a graduate student, the new system extends the bandwidth and sensitivity of the 26-m Telescope by an order of magnitude. This upgrade allows us to use the 26-m Telescope and to make observations for the shorter wavelength part of GMIMS right in our "backyard", but for the longer wavelengths, bigger telescopes need to be used. The 64-m Telescope in Parkes, Australia, for example, is being used to carry out observations in the longer wavelength part of the spectrum covered by GMIMS.

Related Information

Institutes:

• NRC Herzberg Institute of Astrophysics