The Galaxy’s Rosy Glow

NASA Finds Evidence for New Molecular Structure in Space

Polycyclic aromatic hydrocarbons (PAHs) are a class of stable organic molecules made up of carbon and hydrogen. These molecules are flat with carbon atoms in rings of six carbon atoms each.
Credit: NASA

NASA scientists have discovered evidence that a mysterious red glow, seen throughout the Milky Way and other galaxies but never on Earth, radiates from extremely fine dust clusters that cause the glow by combining molecular forces that oppose each other.

Researchers theorize that the red glow, called the Extended Red Emission(ERE), is due to a very unusual form of charged molecular clusters. Measured in billionths of a meter(billionths of a yard), these tiny clusters are made of carbon-rich molecules called polycyclic aromatic hydrocarbons (PAHs) that are chicken-wire shaped. Astronomers have been unable to explain the red glow for more than 30 years, even though PAHs were implicated. The highly luminescent source material requires very harsh ultraviolet radiation, a radiation field so strong that most known polyatomic interstellar molecules would be destroyed. NASA Ames Research Center has been a leader in the study of PAHs under the direction of Ames Astrochemistry Laboratory led by Dr. Louis Allamandola.

We have been studying polycyclic aromatic hydrocarbon molecules (PAHs) in the laboratory at NASA Ames Research Center for a long time, and although I had results that strongly supported the idea that PAHs had something to do with the ERE, the experimental results made it clear that if PAHs were involved, they were present in some as-yet unknown exotic form," said Murthy Gudipati of the University of Maryland and NASA Ames, who recently joined NASAs Jet Propulsion Laboratory after many years of close collaboration with Allamandola.

A mysterious red glow that is present throughout the Milky Way and other galaxies could be due to very unusual, nano-sized, carbon and hydrogen rich particles made of polycyclic aromatic hydrocarbons (PAHs).
Credit: NASA

"These types of highly reactive species are simply not readily accessible for laboratory study, but need very special conditions, added Gudipati. Through a combined effort of laboratory and theoretical chemistry calculations, the current advance in knowledge was made.

Using advanced computational methods, scientists found that the red glow is indeed carried by unusual clusters of polycyclic aromatic hydrocarbon molecules. Highly developed tests confirm the presence of opposing properties within each cluster; they are charged and highly reactive, yet simultaneously, they have a stable, closed-shell electron configuration as does any stable molecule on Earth.

Recent advances in theoretical techniques made it possible to tackle this problem computationally.

Significant difficulties involved in the modeling of charge transfer within large molecular systems required an entirely new approach, said Dr. Timothy Lee, astrochemist and chief of the Space Science and Astrobiology Division at NASA Ames.

Many of the ingredients for life formed in outer space. The Earth formed from star dust, and later meteorites and comets delivered even more materials to our planet. But scientists are still unsure which molecules played the most important roles in life’s origin.
Credit: European Space Agency

"Once we convinced ourselves that our new approach could handle these strange particles, I was able to simulate the detailed emission process on molecular systems much larger than any that had been done before," said Young Min Rhee, postdoctoral fellow at the University of California, Berkeley, and the lead author of the paper published last month in the Proceedings of the National Academy of Sciences. "Our simulation shows that this type of charged PAH cluster can account for the ERE while satisfying the physical requirements necessary to survive the harsh interstellar conditions continued Rhee.

According to scientists, this research has important implications in other areas as well, including combustion processes and exotic nano-materials. For instance, the formation of soot particles produced by diesel and jet engine combustion is not well understood. Self-forming PAH clusters may be the key step to understanding this process. Evidence suggests there is closed-shell charged PAH ions in flames, and the highly robust yet unusual closed-shell PAH clusters described here may be the soot nucleation sites in flames, a result that has been long anticipated. In addition, the research yields fundamental information about how complex molecules containing carbon can persist in space. Although the PAHs concerned in the study are not found on Earth and are not a type of organic molecule used by life as we know it, understanding the chemistry behind such molecules in space can yield clues about how organics might be formed and subsequently delivered to habitable environments in the Universe. Determining what raw materials for life can be produced by chemical evolution in space is an important goal of astrobiologists today.

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