Study Calls 'DIBs' on Solving Astronomical Mystery


New observations by the Gemini North telescope in Hawaii could shed light on an astronomical puzzle dating back 90 years. Specifically, astronomers have identified 13 new diffuse interstellar bands (DIBs) with the longest wavelengths found to date, providing possible evidence about the presence of large carbon-based organic molecules lurking in interstellar clouds of dust.

But first, a bit of history. One night back in 1919, a young female graduate student named Mary Lea Heger was working at the University of California, Berkeley’s Lick Observatory.

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She noticed an odd set of absorption lines in the spectra of distant binary star systems through the telescope, and concluded they were interstellar in origin. Indeed, this finding was the basis for her PhD thesis, completed in 1924.

(Heger was an anomaly in that era, when few women found their way into science, and she left the field after marrying fellow astronomer C. Donald Shane to focus on raising their two children. But she was the quintessential scientific hostess when Shane was named director of the Lick Observatory after World War II. So she remained active on the sidelines of astronomy.)

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Since then, some 500 bands have been spotted by astronomers, mostly in the visible and near-infrared wavelengths. The newly discovered bands fall into the microwave regime, the lowest energy absorption found to date.

Why are DIBs so fascinating to astronomers? Well, they don’t really know what’s causing them. See, stars emit light (radiation) from the ultraviolet regime all the way down to the infrared, but when we observe that light, the entire range might not be visible to us.

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That’s because dust, gas clouds, or other things that make up the interstellar medium are absorbing some of the light at specific wavelengths. Which wavelengths are absorbed depends on which elements are present in the medium. Each element has its own “signature.”

It’s clear DIBs operate on the same principle. They are connected to the interstellar medium, and not a particular kind of star, since they show up around stars of varying ages and sizes. Also, while the light from a star shifts relative to the Earth’s motion, the bands do not.

This is where things get weird. Usually, with the interstellar medium, you’d see very narrow absorption peaks, due to the presence of just a few elements, or even just one element. That’s not the case with DIBs. The absorption spectra are diffuse (hence the name) and don’t appear to correspond to any known element or chemical.

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About all astronomers can say is that whatever is causing DIBs seems to fit into the gas phase, because the absorption wavelengths are pretty constant, and it is molecular rather than atomic in nature because the absorption spectrum is so broad.

This is considered the longest-standing problem in spectroscopy by some astronomers, and the best guess to date is that DIBs are produced by some mix of complex carbon-based compounds similar to amino acids — despite the fact that the interstellar medium is a pretty harsh environment for complex molecules, especially near the galactic center.

That’s where Gemini North astronomers found the 13 DIBs just announced. And once again, the spectral lines don’t match any known signature of simple molecules. However, because these bands are such low-energy, those microwave regimes must now be incorporated into working theoretical models.

It’s possible that scientists might one day be able to use laboratory spectroscopy to identify the composition of DIBs in the infrared regime. For now, it’s not possible to recreate DIBs in the lab, because the required pressures and temperatures are too extreme.

If scientists ever manage to surmount those obstacles, and do indeed find complex carbon-based organic compounds, it would support the notion that the seeds of life were sown first in space and then dispersed onto planets like Earth. And that would be exciting, indeed.

Photos: (top) NASA. (bottom) Gemini Observatory.

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