One of the fun things about covering so many different areas of science is that you often come across things you had no idea existed. This week's discovery goes by the name of "diffuse interstellar bands," a phenomenon first discovered back in the 1920s. Since then, dozens of these DIBs, which represent areas of the spectrum that are absorbed by the interstellar medium, have been discovered. Despite these discoveries—and this week's Nature describes a few more of the bands—we really don't know what's causing them.

Most stars emit radiation over a very broad spectrum, from the ultraviolet well down past the infrared. However, if there's some material between us and the star—say, for example, a cloud of hydrogen—then it can absorb some of that radiation. But it won't absorb it evenly. Instead, each element or compound absorbs a specific set of wavelengths that correspond to the energy gaps between its ground state electrons and their excited states. So a cloud of hydrogen that sits between us and a star will cause some very narrow gaps in the spectrum of the star when we observe it.

Diffuse interstellar gaps seem to operate on this principle, except they're a bit weirder. To start with, they're not the very narrow absorption peaks that typically result from a single element or a small mixture of them—hence the "diffuse" in their name. For another, there are dozens of them now cataloged, and none of them correspond to any known absorption pattern of an element or simple chemical. Finally, the strength of a single band will rarely correlate with the strength of any others, meaning that there must be a mixture of chemicals doing the absorbing, with the quantities of each varying depending on where we're actually doing the observing.

Whatever chemicals are causing this, they don't seem to be associated with the stars themselves. DIBs have been found associated with stars of different ages and sizes, indicating they're unrelated to the stars' history. If the light from a star is Doppler shifted due to motion relative to the Earth, the bands remain stationary, also indicating they're not associated with the star.

DIBs, however, do appear to be associated with the interstellar medium. This material causes a broad decrease in the amount of light we receive from a star, a decline that's present across wide areas of the spectrum. The DIBs are simply narrow areas of the spectrum where the loss of light is much greater.

The new paper describes the first report of DIBs in the microwave area of the spectrum, the lowest energy absorption yet discovered. Naturally, they don't correspond to any known compounds, either. The authors have pinpointed the area where the DIB-generating material resides, and find that it's right near the galactic center, in a region called the Central Molecular Zone. Analysis using the absorption of H 3 + ions (something else I didn't know existed) indicates that the Central Molecular Zone contains huge quantities of turbulent gas, which causes the spectral features to broaden due to Doppler shifting. That neatly explains the diffuse nature of the DIBs.

The galactic center is a much harsher place than we've previously detected any DIBs. It reaches temperatures up to 200K higher and is exposed to much higher levels of cosmic rays. And there's some evidence that this has changed the chemistry of whatever is doing the absorbing, since the ratio of different DIBs at the galactic center is different from ones spotted in cooler locations. Still, it's a rather harsh environment for complex molecules.

Although this tells us quite a bit more about DIBs (a lot more, in my case), it still doesn't tell us what produces them. Our best guess is that they're generated by a mix of complex carbon-based compounds, but we don't know which ones, and it's not obvious how to go about figuring that out.

Nature, 2011. DOI: 10.1038/nature10527 (About DOIs).