Mysterious Glow from the Milky Way Decoded

Nasarevealsk Scientists have been puzzled by a mysterious infrared glow from the Milky Way and other galaxies, radiating from dusty regions in deep space. By duplicating the harsh conditions of space in their laboratories and computers, scientists have identified the mystifying infrared emiters as PAHs that in space are probably produced by carbon-rich, giant stars. "A similar process produces soots here on Earth,” said Louis Allamandola, an astrochemistry researcher at NASA’s Ames Research Center, Moffett Field, Calif.  PAHs are now known to be abundant throughout the universe, but in exotic forms not readily found on Earth.

This mysterious infrared radiation from interstellar space was discovered in the 1970’s and 1980’s. While the infrared signature hinted that PAHs might be responsible, laboratory spectra of only a handful of small, individual PAHs were available to test this idea. By the mid-1990's, observations showed this infrared emission as surprisingly common and widespread across the universe, implying that the unknown carrier was abundant and important. To better understand PAHs, then thought to be too complex to be present in space, their spectra were measured under astronomical conditions.

To capture their spectra, Allamandola led a team of scientists to measure PAH spectra under simulated astronomical conditions and with computer software. This team consisted of experts in many different fields. "This group made a tremendous effort to make this a reality," said Allamandola. "There are now nearly 700 spectra in the database. Six hundred of these have been theoretically computed, and sixty have been measured in the laboratory. The theoretical spectra span the range from two to 2000 microns, the experimental spectra cover two to 25 microns."

Above image is an interstellar nebula, showing the emission from PAHs in red, some PAH molecular structures and the interstellar PAH infrared signature.

The spectra have given insights into the PAHs in space that were impossible to get any other way. Scientists predict that in the near future these spectra will be especially valuable for interpreting observations made with NASA's new airborne observatory, the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the recently launched European Space Agency's (ESA) Herschel Telescope.

"We will expand the database and tools,” said Christiaan Boersma, a NASA postdoctoral fellow at Ames, who designed and developed many parts of the website and tools. "We now use the database to interpret astronomical observations from star and planet forming regions in our galaxy, the Milky Way, and even other galaxies."

“Initially, our hope was to help interpret the experimental spectra, but over time, our computational capabilities made it possible to study molecules much larger than can be studied in the laboratory,” said Charles Bauschlicher, Jr., a world renowned computational chemist at NASA Ames.

"Thanks to the great sensitivity of the Spitzer Telescope, PAHs are seen across the universe, removing any doubt of the importance of these species,” said Allamandola. 

Casey Kazan via JPL/NASA


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