Searching for Diamond Dust in the Milky Way

A Professor's Perspective on Current Events

Professor Felix Lockman
By Felix J. Lockman, Ph.D.

Diamond dust in clouds of gas throughout the Milky Way?   It may sound unlikely, but that is the conclusion of a team of scientists who used radio telescopes to explore a peculiar and elusive source of radio emission.  And in doing so, they may have solved two different puzzles at once.

For many decades astronomers were confident that they had nailed down the main sources of radio emission from galaxies like the Milky Way.  There was “thermal” radio emission that arises from diverse objects like the Moon and planets, warm interstellar dust and even the remnant of the Big Bang.

The intensity of this emission depends on the temperature of an object, which is why it is called thermal.  Then there is non-thermal emission that arises from the collision of high-energy charged particles, like cosmic rays, with a magnetic field.  Non-thermal emission is observed from the magnetic fields of the Earth and Jupiter as they are hit by charged particles from the Sun, and also arises from supernova remnants and cosmic rays spiraling in the magnetic field of the Milky Way.  The two types of radio emission change intensity differently with frequency, so if you can measure an object at several frequencies it’s easy to distinguish one from the other.  There is also discrete, narrow-band emission from quantum states of atoms and molecules like the 21cm line of hydrogen.  So far, so good.

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But starting around twenty years ago astronomers began to turn up evidence for another kind of astronomical radio signal that got called “Anomalous Microwave Emission” or AME.  This was broadband radio emission clearly not formed in a quantum state of an atom or molecule, yet it looked nothing like either thermal or non-thermal emission.  There was a ready candidate for the source of this emission: interstellar dust grains.  Very tiny bits of solid matter in interstellar space could be spun up by collisions with other particles.  It was predicted that the spinning dust could produce radio waves, maybe the AME.

The story is actually quite a bit more complicated than this, for the simple models of spinning dust proved to be too simple, while for many years the observational evidence for AME was uncertain and sometimes contradictory.

The anomalous emission is so weak, and so unpredictable about where it could be detected in the Milky Way, that many scientists dismissed the measurements as errors.  But over the years data kept accumulating and the skeptics were mostly converted, though our understanding remained highly unsatisfactory.  What exactly was making the AME, and why was it seen in some parts of the Milky Way (and other galaxies) and not elsewhere?

Credit: S. Dagnello, NRAO/AUI/NSF

This is where the other part of the story comes in.  It had been known for some time that certain disks of gas around recently-formed stars showed emission at Infrared wavelengths that was consistent with the presence of nano-diamonds: small crystalline  chunks of carbon no bigger than a millionth of a millimeter.

Similar sized nano-diamonds have been found in meteorites in our solar system, so nature must have no trouble making them.  The new exciting news just announced this week is that a team of scientists using radio telescope in the US and Australia showed was that these dusty disks around new stars are sources of AME, but only if the disks also have the infrared signature of nano-diamonds.

The scientists used two radio telescopes: the Green Bank Telescope in West Virginia, and the Australia Telescope Compact Array, to search for AME from several dozen dusty disks around newly-formed stars.  Although they detected the anomalous emission from only 3 disks, those were also the only disks in the sample that had infrared emission from nano-diamonds.

The implication is that anywhere AME is detected — and it is seen over large regions of the Milky Way — there must be nano-diamonds.  The scientists believe that it takes particular conditions of temperature and density to make the little diamonds produce AME, which is why it is found only in specific locations, but they think that the diamonds are everywhere.  There’s lots more work to do, but it’s possible that we may have finally cracked the case of the Anomalous Microwave Emission, and that the solution lies in vast clouds of tiny, tiny diamonds.

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