Within the realm of the Sun’s heliosphere, magnetometers on many international spacecraft have been measuring magnetic fields for decades, proving that even in a region where gravity dominates, there are zones where magnetic fields govern the physics. Recent observations show that the magnetic field is important on all astronomical scales, from the formation of stars and planets to the evolution of galaxies and clusters. The authors describe how new research is revealing this dramatic magnetic universe.
Now that we understand the importance of the magnetic field on all astronomical scales, it is difficult to see why astronomers have apparently ignored this fundamental force of nature in their endeavours to understand the physics of the universe and the objects therein. One of the reasons is that the magnetic field has been difficult to measure; and if you can’t measure the magnetic field, you can’t model it. As a result, astronomers have had little choice but to gloss over the magnetic field in their models and calculations.
This situation is changing with the new telescopes and techniques that have been deployed recently and astronomers are now confronted with data that are so compelling that they can no longer ignore the role of the magnetic field. For example, high-quality polarisation measurements have provided insights into astrophysical environments from collapsing clouds, where stars and planets are forming, to galactic disks, where hundreds of million of stars are evolving, and galactic centres where enormous black holes reside.
A portion of the Lupus I cloud complex. Color shows the emission by interstellar dust grains observed by Herschel Space Observatory. Streamlines show the associated magnetic fields based on Planck polarization data. The inset shows a SOFIA total intensity image of the low-mass protostellar core (~0.1 parsec) with superposed streamlines, showing that the magnetic fields are beginning to take on the expected hourglass shape.
Astronomers have had little choice but to gloss over the magnetic field in their models and calculations
Our own Sun generates a magnetic field that propagates outward in all directions with the solar wind, forming a magnetic cocoon around our solar system called the heliosphere. Within the last decade we have probed the outer reaches of the heliosphere with the Voyager 1 & 2 spacecraft until they passed through the heliopause and into the interstellar medium (ISM).
Current models suggest that the heliosphere is a bubble that may be more drawn out downstream of the stellar wind than upstream. The large pressure measured at the heliopause, at about 120 Astronomical Units (120 times the Earth-Sun distance), is counterbalanced by the unexpectedly high interstellar magnetic fields and stellar winds as well as much higher fluxes of galactic cosmic rays.
The region just outside the heliopause, known as the heliosheath, is where the magnetic fields and stellar winds are draped or diverted around the heliosphere. In this region, interstellar neutral atoms streaming toward the heliopause are ionised, by collisions or radiation, and then accelerated to even higher energies, creating a population of particles called anomalous cosmic rays. We also observe that the heliopause is not a ‘hard’ boundary since solar-system oxygen leaks through and is found in the heliosheath mixing with the stellar winds. A reverse process, where stellar-wind plasma enters the heliosphere is also taking place.
Our Sun is one of a hundred billion stars travelling through the ISM of the Milky Way Galaxy. What the Voyager spacecraft measured were galactic processes that must be taking place around every mature star that generates magnetic fields and produces protective magnetic bubbles. The Voyager missions have given us a unique perspective on the role of the heliosphere in protecting us from the increased cosmic-ray intensity that exists outside our magnetic bubble.
When the Sun was a young, rapidly rotating star, our heliosphere may have been much more expansive, but as it lost mass via nuclear fusion and the outward flowing solar wind, the rotation slowed down and the magnetic activity decreased. Over the last 4.6 billion years, the Sun has completed nearly twenty revolutions around our galactic centre and has ploughed through widely different interstellar environments. We know that the solar system is entering a very large local interstellar cloud and will be completely immersed in a mere few thousand years. Today we can only predict what might happen as the Sun enters this new environment by continuing to monitor the magnetic bubbles of other stars within interstellar clouds.
Magnetic fields from SOFIA in NGC 1068 are showing over a visible light and x-ray composite image of the galaxy from the Hubble Space Telescope, the Nuclear Spectroscopic Array and the Sloan Digital Sky Survey.
Our own Sun generates a magnetic field that propagates outward in all directions with the solar wind, forming a magnetic cocoon around our solar system called the heliosphere
Dominated by neutral gas, plasma and dust, the magnetised ISM has been studied using optical, infrared (IR) and radio polarisation. Contamination from synchrotron radiation in the radio and extinction in the optical and near-IR make it notoriously difficult to observe dusty star-forming regions in these wavebands. However, charged dust grains tend to align perpendicular to the direction of the immersive magnetic field, so they are the best tracers of the magnetic topology. Since the magnetically aligned dust grains heat up and emit in the far-IR, starlight travelling through dusty, magnetised ISM, therefore, becomes strongly polarised in the far-IR near the peak of thermal dust emission at ~100 μm. The far-IR waveband provides the cleanest measurement of this polarised signal. Starlight travelling through dusty, magnetised ISM becomes strongly polarised in the far-IR near the peak of thermal dust emission at ~100 µm.
Since the signature of the interstellar magnetic field is weak, about 10,000 times weaker than Earth’s magnetic field, measuring the strength in the ISM is a formidable task requiring specialised telescopes. The Stratospheric Observatory for Infrared Astronomy (SOFIA’) High-Resolution Airborne Wideband Camera (HAWC+) is a far-IR polarimeter that astronomers use to probe astronomical magnetic fields. The polarised signal measured by SOFIA/HAWC+ can be used to chart the shape and determine the strength of magnetic fields on the critical subparsec-to-parsec scales of cores and filaments where stars are born. The high spatial resolution provided by the 2.7 m diameter SOFIA telescope and increased sensitivity over previous instruments make HAWC+ the perfect tool to address the role of magnetic fields in star formation and galaxy evolution.
At the scales of dense cores (fractions of a parsec) where individual stars are formed, theory predicts that initially, in a uniform magnetic field, charged particles can only flow along the field lines. Later, the gravitational pull grows strong enough to pinch the field lines, giving rise to a characteristic hourglass shape. Observation of such features are rare and had been limited to high-mass stars. However, during its southern-hemisphere deployment in 2018, SOFIA made the first detection of the hourglass feature in a low-mass (Sun-like) protostar. Similar features have now been observed by SOFIA on larger (1-100 pc) scales of the Orion star-forming region and a super star cluster in 30 Doradus, more than 10 and 1000 times bigger than the dense cores, respectively.
Stars form predominantly in cold, dense and extremely filamentary clouds. Observations from the Planck satellite of large-scale magnetic fields revealed that in low-density outskirts of these filaments, matter flows in the direction of the magnetic field, whereas in the dense central regions, the matter flows perpendicular to the direction of the magnetic field. Magnetic fields are playing an important role in shaping these filaments. But for stars to form, these filaments will need to collapse. SOFIA observations help us determine where and how this happens.
The Serpens South Cluster is a young, nearby star forming region that sits at the centre of a network of dense filaments. The extreme youth and proximity of this system makes it an ideal laboratory for testing the role of magnetic fields in a filamentary dark cloud in an early stage of star cluster formation. SOFIA made a critical discovery that in the most opaque parts of the Serpens filaments, the magnetic field once again becomes parallel to the flow of gas, allowing gravitational collapse to occur even in the presence of relatively strong magnetic fields. Such studies are in their infancy, and SOFIA will be making these critical measurements in dozens of filaments over the next year.
Magnetic field streamlines detected by SOFIA are shown over an image of the Whirlpool Galaxy (M51) from NASA’s Hubble Space Telescope.
Galactic magnetic structure
The Voyager missions have given us a unique perspective on the role of the heliosphere in protecting us from the increased cosmic-ray intensity that exists outside our magnetic bubble
Magnetic fields appear to arise in the infant universe from inhomogeneities and anisotropies of electric charges. Theory suggests that the amplification of this ‘seed’ field, for example via galaxy formation, mergers, accretion flows and supernovae explosions, as well as the feedback associated with all these processes, is required to produce the field we observe today. This field appears to be strong enough to regulate star formation, affect the global kinematics of the gas, explain the transport of cosmic rays and even modify the rotation curve. Understanding the origin, amplification and morphology of the magnetic fields is crucial to forming a complete picture of galaxy development. Multi-wavelength observations of different galaxy types are necessary to get a more comprehensive picture of how magnetic fields influence the formation and evolution of galaxies.
SOFIA astronomers have measured, for the first time, the magnetic field tracing the star forming regions along the spiral arms of NGC 1068, the nearest grand-design spiral with an active galactic nucleus and an almost face-on disk. They were able to combine these new observations with other tracers to confirm an aspect of the density wave theory, which predicts that stars form in the arms as gas moves into the wave and is compressed by its gravitational potential. Under this scenario, the spiral arms should look slightly different for distinct tracers because they appear at different phases of the wave. The spiral pattern traces existing stars in the optical, the inter-arm diffuse medium in the radio and ongoing star formation in the far-IR, which is the component seen by HAWC+.
The observations of Messier 51, the Whirlpool Galaxy, add a layer of complexity. The magnetic field lines in the inner region of the galaxy show a regular spiral structure, but field lines in the dense molecular material decouple from those of the diffuse gas in the outskirts. The HAWC+ observations shows a strong distortion and large differences in their orientation with respect to the structure obtained in the radio band. This decoupling might be related to the gravitational interaction with the small companion, Messier 51b, but, strikingly, this effect is not found in the inter-arm region where the gas density is much lower and fewer stars are forming.
Magnetic fields from SOFIA are shown as streamlines over a composite image of Centaurus A taken by the European Southern Observatory (ESO), Atacama Pathfinder Experiment (APEX), Chandra X-Ray observatory, and Spitzer Space Telescope.
The observed differences between both tracers of the magnetic field support the presence of small-scale magnetic dynamos. When combined with galactic rotation and shear forces, these dynamos would help to create the striking spiral patterns visible in the magnetic field structure and support the presence of spiral density waves, which would be compressing the magnetic field lines as the morphological spiral arms move through the galaxy.
The solar system is entering a very large local interstellar cloud and will be completely immersed in a mere few thousand years
Centaurus A is the remnant of a merger between an elliptical and a spiral galaxy that took place about 160 million years ago. HAWC+ observations show that the magnetic field orientation is tightly aligned with the warped molecular aligned with the 3 kiloparsec warped molecular disk.
Although magnetic fields were key to shaping the early universe, they were originally weak and needed to grow stronger over time, and galactic mergers appear to be one of the strengthening mechanisms. As the galactic collision triggered a burst of star formation and reshaped the original spiral galaxy, they combined with gravitational forces to distort, twist and amplify the smaller-scale magnetic fields. Similar merging processes in the early universe may have transformed relatively weak primordial magnetic fields into the powerful forces observed today that affect how galaxies and stars are created.
Observations of Messier 82, a canonical starburst galaxy, reveal a bipolar superwind that originates in the core and extends out into the halo and beyond (~10 kiloparsec). Early observations from HAWC+ show that the geometry of the field at the base of the superwind is perpendicular to the plane of the galaxy, consistent with a scenario where the outflow is dragging the field along with it. SOFIA astronomers used a well-tested technique borrowed from heliophysics – the potential field extrapolation – to determine that these magnetic field lines extend forever, channelling matter enriched with elements like carbon and oxygen into intergalactic space and magnetising the intergalactic medium.
SOFIA results from these relatively nearby galaxies are helping astronomers piece together a coherent picture of how the magnetic fields amplified by turbulent gas motions arising from galactic outflows, mergers and active galaxies have evolved over cosmic time, especially during the violent, feedback-dominated early universe.
Magnetic fields lines in Messier 82 overlaid on a visible and infrared composite image from the Hubble Space Telescope and the Spitzer Space Telescope. The galactic superwind from the central starburst is blasting out plumes of hot gas (red) and a halo of smoky dust (gold) perpendicular to the edge-on galaxy (white).
The Magnetic Universe is an emerging field enabled by new observational techniques, both remote sensing observations and in situ space missions currently being planned. In the far-IR, detailed characterisation of remote magnetic fields can now be obtained, illustrating how they affect and influence different astrophysical environments on different spatial scales. This means that astronomers can no longer ignore the magnetic field in their models and calculations.
A new mission concept is emerging that will explore the boundaries of our own heliosphere and measure the characteristics of the ISM in detail. NASA’s Interstellar Probe will have a nominal lifetime of 50 years and a goal to reach 1000 Astronomical Units. By exploring the heliosphere and ISM, Interstellar Probe will ultimately allow us to better understand how our star interacts with its galactic neighbourhood.
Understanding the origin, amplification and morphology of magnetic fields is crucial to forming a complete picture of galaxy development
Until then, upcoming observations from SOFIA will continue to make unique contributions and refine our knowledge. Recent results from SOFIA have now firmly established that the far-IR polarised signal from magnetically aligned dust grains can be used to determine the strength and structure of the magnetic field from sub-parsec (star formation) to kilo-parsec (galaxy) scales.
These pioneering results are even now leading to a rich legacy as HAWC+ continues to probe the role of the magnetic field in regulating the star formation process, constraining the mechanical energy output from stellar winds and combining these seemingly diverse processes to probe the connections in other galaxies between the gravitational interaction, star formation, spiral density waves and the nature of magnetic fields. We still have much to learn about our place in the universe and how it will evolve over cosmic time, but these new approaches are the way forward.
About the authors
Dr James Green is a magnetospheric scientist. He is currently the NASA Chief Scientist and was previously the director of the Planetary Science Division at NASA Headquarters. He received his PhD in Space Physics from the University of Iowa in 1979 and has also worked at Goddard Space Flight Center and Marshall Space Flight Center.
Dr Naseem Rangwala is an astrophysicist at NASA Ames Research Center and the NASA Project Scientist for the SOFIA airborne observatory. She earned her PhD in physics and astronomy from Rutgers University in New Jersey. Her research focuses on observational astrochemistry and characterising the molecular interstellar medium of galaxies using submillimetre and infrared observations.
Dr Joan Schmelz is Director of the NASA Postdoctoral Program at Universities Space Research Association (USRA), Senior Vice President of the American Astronomical Society and a former chair of the Committee on the Status of Women in Astronomy. She was previously the Associate Director for Science & Public Outreach at SOFIA and the deputy director of the Arecibo Observatory in Puerto Rico. Her research involves observations of solar coronal loops and developing constraints for coronal heating models.