One of the major discoveries made by astronomers over the past 25 years is that polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by astrochemical standards, are common and very abundant throughout much of the Universe. Astronomers discovered PAHs by the infrared emission they give off as they cool down after they are heated by starlight. PAHs, common on Earth in soots, are flat, chicken-wire shaped, nano-sized molecules. The PAHs in space are most likely made in carbon-rich, red-giant stars. Because the infrared emission from PAHs is so common, it can be used to study areas all across the Universe - from regions where stars and planets are forming, to those where they are dying, in the intervening interstellar medium and even extragalactic halos. Figure 1 shows an example of the PAH infrared signature and some PAH structures with the Iris nebula in the background.
A little history. Astronomical observations in the 1970's and 1980's discovered mysterious infrared radiation from space. While this new infrared ‘signature’ hinted that PAHs might be responsible, bona-fide spectra of only a handful of small, individual PAHs were available to test this idea. And these were only for neutral PAHs, not for PAHs as they were expected in space: electrically charged, free, very cold, individual molecules in the gas. A far cry from how they are on Earth. Since PAHs are very sticky molecules, on Earth they are usually stuck together in sooty solids; not charged, not free, not cold, and not in the gas. By the mid-1990's, observations showed this infrared emission was common and widespread across the Universe implying that the unknown carrier was abundant and important. The need for PAH spectra measured under astronomical conditions was critical to test the idea that PAHs are indeed responsible and, if correct, analyze the emission.
To gain this information, starting in the early 1990's, we duplicated the harsh conditions of cold interstellar space in our laboratories and computers, and built up a very large collection of polycyclic aromatic hydrocarbon (PAH) spectra. This collection (Figure 2) is now available on the web at www.astrochem.org/pahdb. Thanks in great measure to this spectral collection, PAHs are now thought to be widespread and common across the Universe and astronomers see the PAH signature almost everywhere they look.
Because it is so common, the infrared signature from PAHs holds the promise to develop into a very important, new 21st century astronomical probe. However, developing PAH spectra into a powerful analytical tool has been seriously hindered because of the complexity in simultaneously handling such large amounts of astronomical and spectroscopic data.
"Because of the wealth of information contained in these spectra, we have worked very hard to develop a 'blind' computational approach to solve this problem. We have also developed it in such a way that others who are interested can also use it," said Dr. Christiaan Boersma the lead author. Boersma continued, "This way the treasure trove of knowledge that is hidden in the spectra of thousands of astronomical objects across the sky gets tapped. Just as with writing, by comparing the spectral signature from each of these different spots on the map with the PAH signatures in the database, we are able to trace how PAHs evolve across space and time. Their electrical charge, their size and shape all change depending on where they are. Bringing all this information to bear on this cosmic emission has revealed the types and amounts of different PAHs present in space and how they evolve from their birth site in red giant stars, to the interstellar medium between the stars, and ultimately into star-forming regions and proto-planetary disks. Now being able to do this completely on a computer represents a major step forward."
Maps of the tell-tale PAH signature across many regions in space were made by NASA’s Spitzer Space Telescope. A particularly detailed map was made of the refelction nebula known as the IRIS nebula. We analyzed these maps with an algorithm-driven, blind-computational approach exclusively using the spectra in the database and traced how the PAHs changed significantly to adjust to the different environment at each spot in the map. Figure 3 shows two different PAH IR signatures at different places in the nebula and Figure 4 the striking changes in the PAHs at different spots in the nebula.
"The spectra in the database have given insights into the composition of the PAHs in space that were impossible to obtain any other way. 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 James Webb Space Telescope (JWST) to be launched later this decade," explained Dr. Jesse Bregman, a member of the team. He continued, "These telescopes are opening new windows on the sky and will enable The Golden Age of IR astronomy and astrophysics to take a giant leap forward because they will measure PAH spectra in greater detail and sensitivity than ever before. Being able to perform blind computational analysis of the infrared spectra they measure using real PAH infrared signatures is an enormous plus."
The database and tools are available on the web at www.astrochemistry.org/pahdb.
A paper describing these results will be published in the Astrophysical Journal on May 14nd, 2013, vol. 769 (2), 117.
This work was supported by NASA's Carbon in the Galaxy Consortium under the auspices of the Astrophysics Research and Analysis Program.