How radio astronomy sees magnetic fields

Newswise – Many objects in the universe have a magnetic field. Planets like Earth and Jupiter, the Sun and other stars, even galaxies billions of light years away. But these magnetic fields typically don't emit light that astronomers can see, even in radio. So how do astronomers study the magnetic fields of distant stars and galaxies?

Although magnetic fields do not emit light, charged particles moving through these magnetic fields often do. For example, auroras on Earth are caused by charged particles in the solar wind that are trapped by Earth's magnetic field. They spiral along the magnetic field lines until they enter our atmosphere near the north and south magnetic poles, which can produce both visible and radio light. We can see the aurora on Earth and Jupiter as a beautiful curtain of colors. Astronomers have observed The radio glow of a brown dwarf aurora.

When magnetic fields are very strong, charged particles caught in these fields can be accelerated to incredible speeds. As they accelerate around a magnetic field, the charges can emit light directly. It is known as synchrotron radiation and is often seen from the hot accretion disks of black holes. Astronomers can use synchrotron radiation to measure how fast the charges are moving and how strong the magnetic field is. This has helped us understand how black holes can rip apart and swallow stars, and has allowed astronomers to determine the size of distant black holes.

Astronomers can also map weak magnetic fields. The Milky Way's magnetic field is not as strong as Earth's, but it extends throughout our galaxy. Our galaxy is full of charged particles in the form of ionized interstellar gas. This ionized gas itself does not emit much light, but it does affect the light that passes through it, especially polarized light such as that emitted by pulsars. When polarized light passes through an ionized gas, its orientation rotates. The amount of polarization rotation depends on the frequency of the light. By comparing the polarization of pulsar light at different frequencies, astronomers can Map of ionized gas distribution in the galaxy. And because this gas aligns with the galactic magnetic field, they can map the field.

We can even measure it The magnetic field of a galaxy billions of light years away. Recently, the Atacama Large Millimeter/Submillimeter Array (ALMA) measured the magnetic field of a galaxy so far away that its light took 11 billion years to reach us. This galaxy is particularly dusty, so ALMA observed the light reflected and emitted by this dust. This light is polarized along the orientation of the dust grains, and because the dust grains travel along the magnetic field lines, astronomers can use this to map the galaxy's magnetic field. It is the most distant galaxy with a magnetic field.

Astronomers don't always need to see something to know it's there. They just need to see the impact they have on what they can see. From dark matter and dark energy to black holes and magnetic fields, radio astronomy helps bring these unseen things to light.