New Data from a Quasar May Provide Insight on the Earliest Stars in the Universe
A recent observation of the chemical properties of the most distant quasar have led some astrophysicists to revise their guess on what the earliest stars were like.
Quasars, which are extremely distant celestial objects that emit a profuse amount of energy, can provide astrophysicists with critical information on the evolution of the early universe. Astrophysicist Yuzuru Yoshii from the University of Tokyo and his research team published their study of the most distant quasar last month in The Astrophysical Journal. Yoshii and his colleagues analyzed the spectral data on this quasar (about 13.1 billion light-years away) from the Gemini North telescope in Hawaii. Their findings challenge the current theories of supernovae and suggest that an even more cataclysmic event is in the works.
A supernova is a potent explosion that happens when massive stars reach the final stage of their life cycle. Its searing temperatures can allow a rapid fusion of elements into heavy metals. Yoshii and his team propose that a new, more powerful supernova contributed to the quasar’s metallicity, and the best-fit candidate happens to include Population III stars (the first stars born in the universe with virtually no metals).
Astronomers have yet to observe a Pop III star. Yoshii and his team were hunting for evidence of these stars, and they noticed a potential clue when looking into a Near-Infrared (NIR) spectrum of the most distant quasar. Since remote celestial objects are so far away, the energy they emit shifts to lower frequencies (longer wavelengths) — like infrared — once it reaches the Earth. Astrophysicists use these signals to determine the chemical composition of an object by spotting emission lines in its spectroscopy. Each element has a unique emission line, hence revealing their presence in the scan.
Stars with less than 8 times the mass of the sun do not produce heavy metals, but stars more massive than that can produce iron, which will lead to a supernova that can produce an abundance of even heavier metals. The aftermath is either a neutron star (a highly compact core of a massive star) or a black hole. A star well over 100 times the mass of the sun may instead end its life as a pair-instability supernova (PISN). This theoretical model would be magnitudes brighter than a supernova and leave no remnant of the star behind. Its higher temperatures would also provide an even more generous pool of heavy metals into space.
Yoshii and his team studied the metallicity of the most distant quasar from its NIR spectroscopy and determined that its iron to magnesium ratio was inconsistent with current observed models of other supernovae. They propose that an explosion far greater could have been involved, such as that of a Pop III star from 150 to 300 times the mass of the sun.
“It was obvious to me that the supernova candidate for this would be a PISN from a Pop III star,” said Yoshii. “I was delighted and somewhat surprised to find that a PISN of a star with a mass about 300 times that of the Sun provides a ratio of magnesium to iron that agrees with the low value we derived for the quasar.”
Yoshii and his colleagues have predicted that a Pop III star of 280 solar masses could have contributed to the quasar’s metallicity; anything too far from this estimated value would not account for the current observation.
Astrophysicist Evan Scannapieco from Arizona State University confirmed the peculiarity of the quasar’s chemical makeup. “It’s like a big lump of iron. That’s really kind of remarkable. The interpretation [could be] consistent with some models of these early stars, but they’re pretty uncertain. Just the fact that you have something that’s 40% iron is crazy. It’s just crazy.”
The mystery remains unsolved, but Yoshii says one way forward is to measure the silicon to iron ratio, which can validate the PISN hypothesis of a star exceeding 300 solar masses. As of now, the answer to how massive the earliest stars were remains open-ended, but perhaps not for long.
Source
Yoshii, Y., Sameshima, H., Tsujimoto, T., Shigeyama, T., Beers, T. C., and Peterson, B. A. (2022) “Potential signature of Population III pair-instability supernova ejecta in the BLR gas of the most distant quasar at z = 7.54∗.” Published in the Astrophysical Journal. DOI: https://doi.org/10.3847/1538-4357/ac8163
