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Astronomers Illuminate Giant Magnetic Ropes in the Sky

This time of year, people in many parts of the world are digging out boxes of holiday light, untying and untwisting the strands to create beautiful, sometimes tacky, glowing displays. Throughout the season, people will gather in front of the best displays, maneuvering around one another to get a better view of the twinkling shows.

25 million light-years away, astronomers have captured another brilliant display. We can’t see it with our eyes, but astronomers with the international CHANG-ES* collaboration have maneuvered telescopes and carefully processed data to make it visible. What they’ve found could shed light on the behaviors of galaxies like our own Milky Way, and might help us better understand some mysterious large-scale features of the universe.


Composite image of the Whale Galaxy revealing large magnetic structures. Credit: Composite image by Jayanne English of the University of Manitoba, with NRAO VLA radio data from Silvia Carolina Mora-Partiarroyo and Marita Krause of the Max-Planck Institute for Radio Astronomy. The observations are part of the project Continuum HAlos in Nearby Galaxies – an EVLA Survey (CHANG-ES). Optical data is from the Mayall 4-meter telescope, collected by Maria Patterson and Rene Walterbos of New Mexico State University. Arpad Miskolczi of the University of Bochum provided the software code for tracing the magnetic field lines.
The whale galaxy is a spiral galaxy, although it looks like a long smudge through Earth-based telescopes because we see it from the side. Like all spiral galaxies, the whale is surrounded by a glowing halo–a less dense region of gas and stars. We know that spiral galaxies emit gas, cosmic rays, and light into intergalactic space through their halos, but we don’t really know how. That’s where CHANG-ES scientists come in. The team is studying the halos of 35 edge-on spiral galaxies using the Karl G. Jansky Expanded Very Large Array (EVLA)—a collection of 27 radio antennas in the desert of New Mexico–to find out what’s happening there.

When most people think of telescopes, they picture telescopes that capture and magnify visible light from objects in the night sky. But galaxies emit electromagnetic radiation over a wide range of wavelengths, including wavelengths not visible to human eyes. Many astronomical objects, including spiral galaxies, emit radio waves. These signals contain information on the structure, composition, and motion of objects, so studying objects in this “light” can tell us things that we can’t learn from studying them in visible light. In addition, radio waves can be captured day or night and aren’t affected by clouds.

EVLA is a giant radio telescope composed of 27 antennas that all work together. Each antenna is a large dish containing 8 receivers. The antennas can be precisely pointed toward the part of the sky you’re interested in and, to make them even more versatile, they can be positioned close together or far apart, depending on the level of detail you need. The further apart they are, the more detail you can capture. The closer together, the better you can capture large-scale features.

The Very Large Array near Socorro, New Mexico, United States. Credit: Hajor (CC BY-SA 2.0).
Radio signals don’t change much as they travel through space, which is one of the reasons radio astronomy is so powerful. But because radio waves are a type of electromagnetic wave, their polarization–a characteristic describing the behavior of the electric field–is influenced by interactions with strong magnetic fields like that of planets and stars. In order to illuminate the structure of the magnetic fields in the halo of the whale galaxy, the researchers first needed to measure the polarization of the radio signals in high resolution, and then determine how the polarizations were influenced by the magnetic forces they experienced.

The researchers collected radio signals coming from the whale galaxy with EVLA in several different configurations in order to map both fine details and large-scale structures. Then, using highly specialized software, they determined the polarizations and how they were influenced by the halo. From this, the researchers recreated the magnetic structure of the halo.

The result wasn’t just scientifically interesting, it was also beautiful. The team detected a large-scale, smooth magnetic pattern in the halo of the spiral galaxy, illustrated by the blue and green bursts in the composite picture. The green lines indicate when the magnetic field is pointing toward the Earth and the blue lines where it is pointing away from the Earth. The researchers say they this field be described by giant magnetic ropes with alternating directions—as you can see by the color pattern, the magnetic field above the disc reverses direction several times.

Close up view of the composite pictures. Credit: See above.
According to the researchers, these magnetic fields are probably generated by the large-scale rotation of gas inside the galaxy. Interstellar gas contains charged particles that, when in motion, create a magnetic field. The magnetic field reinforces the motion of the particles, which reinforces the field, and so on. This is called dynamo action; it’s the same basic process that generates the magnetic field of the Earth and stars.

Magnetic fields impact the evolution and structure of galaxies, the behavior of cosmic rays, and star formation, according to Marita Krause, a researcher from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and one of the leaders of this study. But our knowledge of how and why is incomplete.

Krause says that if you were to simulate the evolution of the universe, starting from the big bang and running through galaxy formation, magnetic fields would absolutely influence the outcome. But we don’t know exactly how because it’s mathematically difficult to include magnetic fields in these simulations. “We are looking at the fields because they are there and they should be understood,” says Krause. “They cannot be neglected because it’s difficult to calculate.”

This is the first time astronomers have detected large magnetic field reversals - that switch between green and blue lines - in the halo of a spiral galaxy, but that doesn’t mean the whale galaxy is unique. It’s the first galaxy halo studied at this sensitivity and with these sophisticated techniques. With 34 similar studies underway, we’ll soon have a sense of just how common these large, twisted magnetic structures are among spiral galaxies. We may even have a display of our own emanating from the Milky Way.

This research was recently published in the journal Astronomy & Astrophysics.

*CHANG-ES stands for the Continuum HAlos in Nearby Galaxies — an EVLA Survey, a collaboration led by Judith Irwin at Queen’s University in Canada

--Kendra Redmond

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