Stellar relics: In the oldest known quasar, astronomers have discovered possible traces of the first generation of stars in our universe. They show up in an unusual excess of iron in the quasar spectrum, which cannot be explained with current models. This chemical fingerprint might come from the explosion of a nearly 300 solar-mass Population III star — a first-generation star that exploded in an exotic pair-instability supernova.
Die first stars of our universe a few hundred million years following the Big Bang. These Population III stars not only ended the “dark ages“, but also created the first heavier elements. When the star giants, which weighed hundreds to thousands of solar masses, exploded in supernovae following a few million years, they distributed the heavy elements in the cosmos and also enriched neighboring star cradles with the material.
Manhunt in the light of the oldest quasar
Now astronomers may have discovered the spectral signal of this primordial debris for the first time – in the light of oldest known quasar ULAS J1342+0928. This active supermassive black hole shows a redshift of z=7.45 and thus existed when the Universe was 690 million years old. To find out more regarding this quasar and its surroundings, the University of Tokyo’s Yuzuru Yoshii and colleagues analyzed its light using the near-infrared spectrograph on the Gemini North telescope in Hawaii.
Their hope: Because an active black hole attracts gases from its surroundings and causes them to glow, the quasar light spectrum must reflect the composition of its “feed” – and thus also the possible element signature of primordial stellar remnants. “This is the first time that a detailed near-infrared spectrum of the most distant quasar has been obtained,” the astronomers explain.
Inexplicably much iron
The surprising result: in the light of the ancient quasar, unusually strong spectral signatures of the element iron were found – an element that was actually rare in the early universe because it was heavy. “The spectrum of this quasar has an iron enrichment that is 20 times higher than in solar conditions,” report Yoshii and his colleagues. For the time just under 700 million years following the Big Bang, this is very unusual.
Where might all this iron have come from? The obvious conclusion would be that this heavy element was created by nuclear fusion in early stars and then released during their supernova. The problem, however, is that common stellar explosions such as the core-collapse supernova of massive stars or the type 1a supernova in binary star systems cannot release such large amounts of iron, as the team has determined using models:
“We find that such unusual enrichment cannot be explained by the standard notion of chemical evolution, which only considers contributions from classical supernovae,” the astronomers say.
Remnant of a pair instability supernova?
This means: The conspicuous iron enrichment must have come from a different type of stellar explosion. “The most promising candidate for this is a pair instability supernova,” explain Yoshii and his colleagues. In this supernova, a star is completely ruptured without leaving a black hole or neutron star. According to current theory, it only occurs in extremely massive stars of more than 150 solar masses, but has never been observed before.
With the help of astrophysical models, the researchers investigated which element signatures such a pair instability supernova would leave behind for different stellar variants. The result: Such a high amount of iron as observed in the quasar fits best with the pair instability supernova of a very first generation star. “We conclude that the magnesium-iron ratio in the quasar ULAS J1342 was caused by the pair instability supernova of a Population III star of 280 solar masses,” the researchers write.
The signature of the very first stars.© NOIRLab
Star birth and death in the early cosmos
If confirmed, astronomers would have found the chemical fingerprints of the very first stars in the universe. According to their scenario, one of these primordial star giants must have formed at the center of a protogalactic blob of gas. After regarding two million years, it then exploded and distributed the heavy elements it had formed in the dense gas around it.
At the same time, at the center of this protogalaxy, the central black hole became active and began to attract and devour some of these gases. A growing ring of hot, radiant plasma formed around the event horizon. As it grew, the black hole gradually transformed into a quasar, releasing such large amounts of radiation that they can still be seen billions of light-years away. Because part of this radiation comes from the relics of the very first stars orbiting the black hole, the signature of the Population III star is also preserved in it – and has now been observed.
So much for Yoshii and his team’s scenario. However, whether this is the case needs to be checked further. As they explain, one way to do this would be to also measure the silicon to iron ratio in the quasar’s spectrum. Because this might reveal whether there was a pair instability supernova or a normal core collapse supernova. (Astrophysical Journal, 2022; two: 10.3847/1538-4357/ac8163)
What: NOIRLab
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