Space glitter: cosmic nanodiamonds may steal some sparkle from a quasar

Natural History, Dec, 2005 by Charles Liu

Interstellar matter--the gas and dust that surround the stars--play all kinds of wacky tricks with light. Sometimes they change the light that shines through them from more distant objects. Sometimes they shine on their own. Some of their light appears to us on Earth in the form of colorful, ethereal nebulae, with equally colorful names: Carina, the Cat's Eye, Rosette, and the Veil.

Yet the same dust and gas that floats in the gaps between the stars--somewhat obscurely dubbed the interstellar medium, or ISM--can blur, distort, and even completely block our view of celestial objects. The interstellar medium comprises only about 1 percent of the mass of our Milky Way Galaxy, including "dark matter," and the bulk of the ISM drifts about in billowing clouds and wispy nebulae in a highly rarefied state, at a density of less than a thousand atoms per cubic inch. Even the best earthbound laboratory vacuums are millions of times denser. Nevertheless, the ISM makes it very hard for us astronomers to know exactly what we're looking at.

Frustrated but undaunted, we infer what lies behind the haze by assuming certain distributions of dust and gas, then modeling their effects on light radiating through them. If the models can be "tuned" to predict what we actually measure, we can cancel out the effects assumed in the model when we analyze the data we gather.

A research team led by Luc Binette, an astronomer at the Universidad Nacional Autonoma de Mexico, in Mexico City, recently studied how the ISM would affect estimates of the energy output of quasars--distant, titanic, gravity-driven energy producers that are powered by supermassive central black holes. Binette's group suggests that a strange feature of the quasars' ultraviolet light, the so-called "big blue bump," could be explained if the light had to pass through a shimmering curtain of diamonds, each less than a millionth of an inch across.

To grasp how diamonds could explain the blue bump, think about what happens around noon on a clear day as the Sun shines through Earth's atmosphere. The air molecules preferentially scatter the blue, shorter-wavelength rays of sunlight in all directions, giving the sky a soft blue glow. Most of the other, longer-wavelength colors of the Sun pass directly through the air, giving the Sun a yellowish cast. At sunset, however, as the Sun moves closer to the horizon, sunlight shines primarily through the lower layers of our atmosphere, where a lot of atmospheric dust is suspended. The dust particles are much bigger than the various kinds of molecules in air, and they absorb much of the blue light and let through the longer-wavelength orange and red light. The combined effects of the air and dust turn the Sun orange and the sky red.

The ISM affects light in deep space the way gas and dust affect light on Earth, and then some. Even though the ISM is sparse compared to the particles in Earth's atmosphere, light that crosses the vast distances of interstellar space must often pass through great quantities--and a great many kinds--of material. When light from astronomical sources is analyzed, the effects of the ISM become evident: bright bands signal light being emitted, dark bands indicate light being absorbed, and the overall distribution of light is distinctly different from that emitted by the original source.

Even without the effects of the ISM, though, the light from quasars is a challenge to interpret. Their light comes not from stars, but rather from the glow of superheated gas swirling into a central black hole. Hence their spectra are quite different from those of ordinary galaxies, particularly at very short, non-visible wavelengths. For example, quasars far outshine ordinary galaxies in ultraviolet and X-ray light.

One particularly prominent feature is that aforementioned big blue bump--a strong, broad enhancement of ultraviolet radiation at wavelengths of about a hundred nanometers (about 0.000004 inch). Most quasar experts think the bump is the result of physical processes around the black hole. Binette's team, however, wondered whether the ISM itself could be responsible: if dust grains a hundred or so nanometers in diameter filled part of the space between us and a quasar, could they transform a smooth source of ultraviolet light into a bumpy spectral signature?

By examining the properties of various kinds of materials, Binette and his colleagues found that dust grains of that size could indeed cause a big blue bump. The grains, however, would have to be made up of a crystalline form of carbon: diamonds. But the amount of "nanodiamond" dust needed to account for the bump is so great that the dust couldn't be evenly spread throughout space; the conditions in the universe have never been right to make so many diamonds. So the nanodiamonds would have to be produced near the quasar itself. A cloud of at least a thousand Earth-masses' worth of them--or, in jewelers' terms, 30 trillion quintillion carats--would have to be floating between the quasar and our viewing location here on Earth.