Polycyclic aromatic hydrocarbons (PAHs) are found on Earth in tar and coal and are closely monitored as a health hazard for humans. But in space, PAHs are part of the dust that makes the regions between stars less of an empty void than it is in our imaginations. More importantly, PAHs are also a leading contender as being a building block for life. They may have played an instrumental role in the primordial ooze, helping to jumpstart biological processes by mediating RNA synthesis. But the dust also performs a more mundane function: it blocks light, and may have tricked astronomers into thinking star formation was less active in the early Universe than indicated by new findings.
Researchers led by graduate student Irene Shivaei at the University of California, Riverside measured this dust in ancient galaxies. They found peak star formation was more active than previously measured, observing approximately 30% higher infrared luminosity 10 billion years ago than measured in previous studies. The results were published on March 15, 2017, in The Astrophysical Journal.
The team used the Keck telescope in Hawaii to observe approximately 1,500 galaxies in visible light. The galaxies were all between 1.5 to 4.5 billion years old, from the era of peak star formation in the early Universe. They incorporated additional infrared spectra from NASA’s Spitzer Space Telescope and the European Space Agency’s Herschel Space Observatory, which allowed them to identify PAH and dust emission in mid- to far-infrared wavelengths.
Telescopes, including the Keck telescope, on Mauna Kea in Hawaii. Credit: Brian Harris
“Despite the ubiquity of PAHs in space, observing them in distant galaxies has been a challenging task,” Shivaei explained in a press release. “A significant part of our knowledge of the properties and amounts of PAHs in other galaxies is limited to the nearby universe.”
Emission of PAHs dropped in low-mass galaxies, which is consistent with other research on similar nearby galaxies. These low-mass galaxies are metal-poor — they have a lower fraction of any atoms heavier than hydrogen or helium — and produce intense radiation. The combination means less carbon production, an essential component of PAHs, and more disruptive events like hard radiation and supernova shockwaves that can destroy PAHs.
Emission also decreased in relatively young galaxies compared to older ones: galaxies between 50 to 600 million years old had a third the PAH intensity of galaxies 900 million years old or older. Researchers aren’t entirely sure of the mechanisms behind how galaxies create PAHs, but a leading theory is that they’re produced by carbon stars, an evolved star where carbon was built up through helium fusion. Therefore, PAHs are expected to be produced in smaller quantities in young galaxies than in older galaxies.
One of the five study regions of star formation and dusty galaxy regions, and the telescopes used in the study. Credit: Mario De Leo-Winkler/Spitzer Space Telescope/NASA/ESA/The Hubble Heritage Team
PAHs are flat hexagonal molecules of hydrogen and carbon, the structure resembling chicken wire. The “aromatic” in their name means the chemical compounds have resonance strengthening the bonds, so the molecules are more stable than other molecules with similar geometry. This is unrelated to — although easily confused with — volatile “aroma compounds” that produce smells. PAHs are actually both, possessing strong resonance bonds and a distinct smell.
While we can’t smell space directly, we have hints of what it might smell like from astronauts sniffing PAHs clinging to their spacesuits after extravehicular activities. In her book What’s It Like in Space? Ariel Waldman reported space smells like a “burnt almond cookie,” “ozone,” “fried steak,” “sweet-smelling welding fumes,” or a mild version of a car engine overheating. The new research indirectly means that the earlier universe may have been smellier than we thought.
PAHs and dust in the early universe are yet another research topic that will benefit from the future James Webb Space Telescope. The infrared telescope, currently slated to launch in October 2018, will be able to observe mid- and far-infrared wavelengths characteristic for PAH and dust emission in greater detail.