Magnetic Fields in Inter-cluster Space: Measured at Last
by Jean Tate
In the heart of an active galaxy, matter falling into a supermassive black hole somehow creates jets of particles traveling near the speed of light. For active galaxies classified as blazars, one of these jets beams right toward Earth. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab
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