Astronomy Without A Telescope – Cosmic Magnetic Fields


The mention of cosmic-scale magnetic fields is still likely to met with an uncomfortable silence in some astronomical circles – and after a bit of foot-shuffling and throat-clearing, the discussion will be moved on to safer topics. But look, they’re out there. They probably do play a role in galaxy evolution, if not galaxy formation – and are certainly a feature of the interstellar medium and the intergalactic medium.

It is expected that the next generation of radio telescopes, such as LOFAR (Low Frequency Array) and the SKA (Square Kilometre Array), will make it possible to map these fields in unprecedented detail – so even if it turns out that cosmic magnetic fields only play a trivial role in large-scale cosmology – it’s at least worth having a look.

At the stellar level, magnetic fields play a key role in star formation, by enabling a protostar to unload angular momentum. Essentially, the protostar’s spin is slowed by magnetic drag against the surrounding accretion disk – which allows the protostar to keep drawing in more mass without spinning itself apart.

At the galactic level, accretion disks around stellar-sized black holes create jets that inject hot ionised material into the interstellar medium – while central supermassive black holes may create jets that inject such material into the intergalactic medium.

Within galaxies, ‘seed’ magnetic fields may arise from the turbulent flow of ionised material, perhaps further stirred up by supernova explosions. In disk galaxies, such seed fields may then be further amplified by a dynamo effect arising from being drawn into the rotational flow of the whole galaxy. Such galactic scale magnetic fields are often seen forming spiral patterns across a disk galaxy, as well as showing some vertical structure within a galactic halo.

It is anticipated that next generation radio telescopes like the Square Kilometre Array will significantly enhance cosmic magnetic field research. Credit Swinburne AP.

Similar seed fields may arise in the intergalactic medium – or at least the intracluster medium. It’s not clear whether the great voids between galactic clusters would contain a sufficient density of charged particles to generate significant magnetic fields.

Seed fields in the intracluster medium might be amplified by a degree of turbulent flow driven by supermassive black hole jets but, in the absence of more data, we might assume that such fields maybe more diffuse and disorganised that those seen within galaxies.

The strength of intracluster magnetic fields averages around 3 x 10-6 gauss (G), which isn’t a lot. The Earth’s magnetic fields averages around 0.5 G and a refrigerator magnet is about 50 G. Nonetheless, these intracluster fields offer the opportunity to trace back past interactions between galaxies or clusters (e.g. collisions or mergers) – and perhaps to determine what role magnetic fields played in the early universe, particularly with respect to the formation of the first stars and galaxies.

Magnetic fields can be indirectly identified through a variety of phenomena:
• Optical light is partly polarised by the presence of dust grains which are drawn into a particular orientation by a magnetic field and then only let through light in a certain plane.
• At a larger scale, Faraday rotation comes into play, where the plane of already polarised light is rotated in the presence of a magnetic field.
• There’s also Zeeman splitting, where spectral lines – which normally identify the presence of elements such as hydrogen – may become split in light that has passed through a magnetic field.

Wide angle or all-sky surveys of synchrotron radiation sources (e.g. pulsars and blazars) allow measurement of a grid of data points, which may undergo Faraday rotation as a result of magnetic fields at the intergalactic or intracluster scale. It is anticipated the high resolution offered by the SKA will enable observations of magnetic fields in the early universe back to a redshift of about z =5, which gives you a view of the universe as it was about 12 billion years ago.

Further reading: Beck, R. Cosmic Magnetic Fields: Observations and Prospects.

A New Class for Tau Scorpii

Many classes of stars are named for an early, distinguished member of a certain type of stars. For example, Cepheid variables take their namesake from the periodic variable Delta Cephei, first recognized by John Goodricke, although Eta Aquillae, another Cepheid, was recognized as a periodic variable with the same period just before Delta Cephei. Since the time of Goodricke’s discovery, many more classes of objects have been discovered from T Tauri, to W Ursa Majoris, to Delta Scorpii.

But sometimes, stars must wait before more members of their class are discovered. Tau Scorpiiis a massive B0 star and one of the rare high mass stars for which magnetic fields have been measured. To distinguish it even further, studies have shown that its magnetic field is unusually complex, being much more tangled than most stars and not showing distinct dipoles. Additionally, this unusual star has been shown to have weaker stellar winds (and consequently, mass loss rates) than most B0 type stars, as well as spectral features that are simultaneously characteristic of stars on the main sequence and young giants. Meanwhile, the star is believed to be only a few million years old. A first step towards characterizing such odd objects is to find more. Fortunately, astronomers have discovered two more stars similar to Tau Scorpii.

The two new stars, HD 66665 and HD 63425, were first recognized as unusual from their spectra, taken by the Canada-France-Hawaii Telescope. Using these spectra, the team, led by Véronique Petit at West Chester University, recognized that these stars had the same peculiar winds as Tau Scorpii. While Petit’s group could not completely constrain the mass loss rates, they did place an upper limit on both, establishing that they too shared the “weak wind problem” in which the expected mass loss rate for such stars was roughly 20 times higher. This prompted the team to investigate each star for magnetic fields.

Although the team wasn’t able to fully analyze the magnetic fields during their observing run to determine just how unusual they were, the team did establish both stars did have magnetic fields present and that they were similar in strength to that of Tau Scorpii. These two pieces of information has led the team to conclude that HD 66665 and HD 63425, along with Tau Scorpii, constitute a new class of stars. Additional confirmation could come from similar conclusions on the age of the analogues.

Petit’s team doesn’t speculate as to the nature of this emerging class in this paper. However, an earlier work of which Petit was a co-author, examined Tau Scorpii specifically. In it, the team examined whether the unusual field was a “frozen in” fossil from formation, or actively produced by an unusual dynamo inside the star. Fields produced by dynamos require large portions of the interior of the star undergoing convection. Models of massive stars predict that convection is likely to be limited in such stars. Another key component is rotation. Tau Scorpii is an extremely slow rotator, so the team concluded that a dynamo is unlikely in this case. As such, the fossil-field theory was more likely. Further investigation of HD 66665 and HD 63425 will certainly be necessary to further compare these stars to Tau Scorpii.

Magnetic Fields in Inter-cluster Space: Measured at Last

How Does Light Travel?

The strength of the magnetic fields here on Earth, on the Sun, in inter-planetary space, on stars in our galaxy (the Milky Way; some of them anyway), in the interstellar medium (ISM) in our galaxy, and in the ISM of other spiral galaxies (some of them anyway) have been measured. But there have been no measurements of the strength of magnetic fields in the space between galaxies (and between clusters of galaxies; the IGM and ICM).

Up till now.

But who cares? What scientific importance does the strength of the IGM and ICM magnetic fields have?

The Large Area Telescope (LAT) on Fermi detects gamma-rays through matter (electrons) and antimatter (positrons) they produce after striking layers of tungsten. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

Estimates of these fields may provide “a clue that there was some fundamental process in the intergalactic medium that made magnetic fields,” says Ellen Zweibel, a theoretical astrophysicist at the University of Wisconsin, Madison. One “top-down” idea is that all of space was somehow left with a slight magnetic field soon after the Big Bang – around the end of inflation, Big Bang Nucleosynthesis, or decoupling of baryonic matter and radiation – and this field grew in strength as stars and galaxies amassed and amplified its intensity. Another, “bottom-up” possibility is that magnetic fields formed initially by the motion of plasma in small objects in the primordial universe, such as stars, and then propagated outward into space.

So how do you estimate the strength of a magnetic field, tens or hundreds of millions of light-years away, in regions of space a looong way from any galaxies (much less clusters of galaxies)? And how do you do this when you expect these fields to be much less than a nanoGauss (nG), perhaps as small as a femtoGauss (fG, which is a millionth of a nanoGauss)? What trick can you use??

A very neat one, one that relies on physics not directly tested in any laboratory, here on Earth, and unlikely to be so tested during the lifetime of anyone reading this today – the production of positron-electron pairs when a high energy gamma ray photon collides with an infrared or microwave one (this can’t be tested in any laboratory, today, because we can’t make gamma rays of sufficiently high energy, and even if we could, they’d collide so rarely with infrared light or microwaves we’d have to wait centuries to see such a pair produced). But blazars produce copious quantities of TeV gamma rays, and in intergalactic space microwave photons are plentiful (that’s what the cosmic microwave background – CMB – is!), and so too are far infrared ones.

MAGIC telescope (Credit: Robert Wagner)

Having been produced, the positron and electron will interact with the CMB, local magnetic fields, other electrons and positrons, etc (the details are rather messy, but were basically worked out some time ago), with the net result that observations of distant, bright sources of TeV gamma rays can set lower limits on the strength of the IGM and ICM through which they travel. Several recent papers report results of such observations, using the Fermi Gamma-Ray Space Telescope, and the MAGIC telescope.

So how strong are these magnetic fields? The various papers give different numbers, from greater than a few tenths of a femtoGauss to greater than a few femtoGauss.

“The fact that they’ve put a lower bound on magnetic fields far out in intergalactic space, not associated with any galaxy or clusters, suggests that there really was some process that acted on very wide scales throughout the universe,” Zweibel says. And that process would have occurred in the early universe, not long after the Big Bang. “These magnetic fields could not have formed recently and would have to have formed in the primordial universe,” says Ruth Durrer, a theoretical physicist at the University of Geneva.

So, perhaps we have yet one more window into the physics of the early universe; hooray!

Sources: Science News, arXiv:1004.1093, arXiv:1003.3884