Surprising substances have been found near supermassive black holes by NASA’s renowned new observatory.
Polycyclic aromatic hydrocarbons (PAHs), which include carbon and are thought to be incapable of surviving at the heart of active galaxies, have been discovered by the James Webb Space Telescope (JWST) in three galaxies. Intriguingly, the observations also imply that the radiation near the supermassive black holes in these galaxies has changed the general characteristics of the PAHs. This could complicate a crucial method astronomers use to assess star formation and could also affect the usefulness of the PAHs as biological building blocks.
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A team of astronomers led by Oxford University’s Ismael Garca-Bernete have examined observations of three active galaxies made by JWST’s Mid-Infrared Instrument (MIRI). The three galaxies are NGC 6552, which is located 370 million light-years from Earth in the constellation Draco; NGC 7319, which is one of the famous Stephan’s Quintet’s five galaxies and is located 311 million light-years away in Pegasus; and NGC 7469, which is also located in Pegasus and is about 200 million light-years away.
PAHs are molecules distinguished by carbon atom rings. These molecules are extremely prevalent across the cosmos, from far-off galaxies to comets in our own solar system. Because of their abundance, they serve as both useful tracers for star formation and potential building blocks for life. In most cases, when astronomers identify PAHs in this method, they can be certain that there are hot, young stars nearby because PAHs emit strongly at infrared wavelengths observable by MIRI when they are irradiated by the ultraviolet radiation in sunlight.
Garca-Bernete wanted to know if the PAH emissions in the active galaxy’s dense, ultraviolet-rich core were the same as those in the spiral arms of galaxies, which are more tranquil star-forming areas. The action of gas descending into a supermassive black hole can also emit torrents of ultraviolet light that cause the PAHs to glow, in addition to the possibility of star formation at the centres of active galaxies.
In accordance with earlier simulations, all PAH molecules would be vaporised by the intense radiation surrounding the supermassive black hole in the centre of an active galaxy. Instead, MIRI found that the centre areas of all three analysed galaxies were rich in PAHs. Observations, however, revealed that the emission originated from bigger, electrically neutral PAH molecules, proving that radiation had actually wiped off smaller, electrically charged PAHs. The team hypothesised that the larger PAH molecules may have survived because they were shielded by thick, engulfing clouds of molecular gas.
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Because star-forming regions are often higher in electrically charged PAHs, the loss of the smaller, electrically charged PAHs is problematic for astronomers who utilise these substances to trace star formation. Astronomers cannot determine where stars might be formed if they are killed in the centres of active galaxies.
The last stage, according to Garca-Bernete, is to evaluate a bigger sample of active galaxies with various characteristics. “This will help us better understand the characteristics of PAH molecules in the nuclear area of galaxies and how they survive there. Such information is essential for accurately measuring the amount of star formation in galaxies and the evolution of galaxies across time using PAHs.”
The study was released in the journal Astronomy and Astrophysics on September 30.
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