Astronomers using the Green Bank Telescope (GBT) at the National Radio Astronomy Observatory (NRAO) have made the first definitive interstellar detection of benzonitrile, an organic molecule that helps to chemically link molecules in the quest to identify the source of a faint IR glow that permeates the Milky Way and other galaxies. 

The faint cosmic light, which presents itself as a series of spikes in the IR spectrum, had no easily identifiable source. It seemed unrelated to any recognizable cosmic feature, such giant interstellar clouds, star-forming regions or supernova remnants. The likely culprit, scientists eventually deduced, was the intrinsic IR emission from a class of organic molecules known as polycyclic aromatic hydrocarbons (PAHs), which, scientists would later discover, are plentiful; nearly 10 percent of all the carbon in the universe is tied up in PAHs. 

Even though, as a group, PAHs seemed to be the answer to this mystery, none of the hundreds of PAH molecules known to exist had ever been conclusively detected in interstellar space. This new data from the National Science Foundation’s (NSF) GBT show, for the first time, the convincing radio fingerprints of a close cousin and chemical precursor to PAHs, the molecule benzonitrile. 

The team, led by chemist Brett McGuire, detected this molecule’s telltale radio signature coming from a nearby star-forming nebula known as the Taurus Molecular Cloud 1 (TCM-1), which is about 430 light-years from Earth. 

“These new radio observations have given us more insights than infrared observations can provide,” McGuire said. “Though we haven’t yet observed polycyclic aromatic hydrocarbons directly, we understand their chemistry quite well. We can now follow the chemical breadcrumbs from simple molecules like benzonitrile to these larger PAHs.” 

Though benzonitrile is one of the simplest so-called aromatic molecules, it is in fact the largest molecule ever seen by radio astronomy. It also is the first six-atom aromatic ring molecule ever detected with a radio telescope. As molecules tumble in the near vacuum of interstellar space, they give off a distinctive signature, a series of telltale spikes that appear in the radio spectrum. Larger and more complex molecules have a correspondingly more complex signature, making them harder to detect. PAHs and other aromatic molecules are even more difficult to detect because they typically form with symmetrical structures. 

Benzonitrile’s lopsided chemical arrangement allowed McGuire and his team to identify nine distinct spikes in the radio spectrum that correspond to the molecule. They also could observe the additional effects of nitrogen atom nuclei on the radio signature. 

“The evidence that the GBT allowed us to amass for this detection is incredible,” said McGuire. “As we look for yet larger and more interesting molecules, we will need the sensitivity of the GBT, which has unique capabilities as a cosmic molecule detector.” 

The Green Bank Observatory and the NRAO are facilities of the NSF, operated under a cooperative agreement by Associated Universities Inc.