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Astronomers have discovered the most distant pair of merging quasars observed just 900 million years after the Big Bang.

This period, known as the Cosmic Dawn, is crucial because it marks the beginning of the formation of stars and galaxies, which led to the reionization of the universe. These quasars provide insight into the formation of supermassive black holes and the early evolution of galaxies, highlighting a significant cosmological transition during the epoch of reionization.

Cosmic expansion and quasar formation

From the very first moment after the Big Bang, the universe has been expanding. This means that the early universe was significantly smaller and early-formed galaxies were more likely to interact and merge. The merging of galaxies fuels the formation of quasars—extremely luminous galactic nuclei where gas and dust falling into a central supermassive black hole emit enormous amounts of light. So when they look back at the early universe, astronomers would expect to find numerous pairs of quasars in close proximity to each other as their host galaxies undergo a merger. However, they were surprised to find exactly none – until now.

A team of astronomers discovered a pair of quasars with a double record. They are not only the most distant pair of merging quasars ever discovered, but also the only pair confirmed in the past era of the earliest formation of the universe.

Discovery of distant merging quasars

Using the Gemini North Telescope, half of the Gemini International Observatory, which is supported in part by the US National Science Foundation and managed by NSF NOIRLaba team of astronomers have discovered a pair of merging quasars observed only 900 million years after Big bang. It is not only the most distant pair of merging quasars ever found, but also the first confirmed pair in the period of the universe’s history known as the Cosmic Dawn.

The Significance of Cosmic Dawn and Reionization

The Cosmic Dawn spans from about 50 million years to one billion years after the Big Bang. During this period, the first stars and galaxies began to appear, filling the dark universe with light for the first time. The appearance of the first stars and galaxies ushered in a new era in the formation of the cosmos, known as the Age of Reionization.

Although we are not sure exactly when the first stars began to shine, we do know that they must have formed sometime after the recombination era when hydrogen and helium atoms formed (380,000 years after the Big Bang) and before the oldest known galaxies exists (400 million years after the big bang). The ultraviolet light emitted by the first stars broke down the neutral hydrogen gas filling the universe into hydrogen ions and free electrons, ushering in the era of reionization and the end of the dark ages of the universe. Courtesy: NASA, ESA, CSA, STScI

The Age of Reionization that took place at the Cosmic Dawn was a period of cosmological transition. Beginning roughly 400 million years after the Big Bang, ultraviolet light from the first stars, galaxies, and quasars spread through space, interacting with the intergalactic medium and stripping the universe’s primordial hydrogen atoms of their electrons in a process known as ionization. The Age of Reionization was a critical era in the history of the universe that marked the end of the cosmic dark ages and laid the groundwork for the large structures we see in our local universe today.

Quasars and the age of reionization

To understand the exact role that quasars played during the epoch of reionization, astronomers are interested in finding and studying quasars that inhabited this early and distant era.

“The statistical properties of quasars in the Reionization Epoch tell us many things, such as the progress and origin of reionization, the formation of supermassive black holes during the Cosmic Dawn, and the earliest evolution of quasar host galaxies,” said Yoshiki Matsuoka, an astronomer. from Ehime University in Japan and lead author of the paper describing these results, published in Astrophysical Journal Letters.

This image taken with the Subaru Telescope’s Hyper Suprime-Cam shows a pair of quasars in the process of merging. The faint patches of red caught the eye of astronomers, and subsequent spectroscopy with the Gemini North telescope, one half of the Gemini International Observatory, which is supported in part by the US National Science Foundation and operated by the NSF NOIRLab, confirmed that these objects were quasars. The pair is seen only 900 million years after the Big Bang. It is not only the most distant pair of merging quasars ever found, but also the first confirmed pair in the period of the universe’s history known as the Cosmic Dawn. Courtesy: NOIRLab/NSF/AURA/TA Rector (University of Alaska Anchorage/NSF NOIRLab), D. de Martin (NSF NOIRLab) and M. Zamani (NSF NOIRLab)

Quasar Pair Revealed

About 300 quasars have been discovered in the age of reionization, but none of them have been found in a pair. It was while Matsuoka and his team were reviewing images taken with the Subaru Telescope’s Hyper Suprime-Cam that a faint red spot caught their eye. “While reviewing images of quasar candidates, I noticed two similar and extremely red sources next to each other,” Matsuoka said. “The discovery was purely accidental.”

The team was not sure they were a pair of quasars because distant quasar candidates are contaminated by numerous other sources, such as foreground stars and galaxies and the effects of gravitational lensing. To confirm the nature of these objects, the team conducted follow-up spectroscopy using the Subaru Telescope’s Faint Object Camera and Spectrograph (FOCAS) and Gemini North’s Gemini Near-Infrared Spectrograph (GNIRS). The spectra, which decompose the light emitted from a source into its wavelengths, obtained with GNIRS, were crucial for characterizing the nature of the quasar pair and their host galaxies.

Consequences of the discovery

“What we learned from the GNIRS observations was that quasars are too faint to be detected in the near-infrared range, even with one of the largest telescopes on earth,” Matsuoka said. This allowed the team to estimate that some of the light detected in the optical wavelength range does not come from the quasars themselves, but from the ongoing star formation taking place in their host galaxies. The team also found that the two black holes are massive, each 100 million times the mass of the Sun. This, combined with the presence of a bridge of gas extending between the two quasars, suggests that they and their host galaxies are undergoing a large-scale merger.^[1]

“The existence of merging quasars in the reionization epoch has been expected for a long time. It has already been confirmed for the first time,” Matsuoka said.^[2]

Future perspectives in quasar research

The age of reionization connects the earliest formation of cosmic structure to the complex universe we observe billions of years later. By studying distant objects from this period, astronomers gain valuable insight into the process of reionization and the formation of the first objects in the universe. More discoveries like this may be on the horizon with the NSF–DOE Vera C. Rubin Observatory’s Decadal Study of Space and Time (LSST) starting in 2025, which is poised to discover millions of quasars using its deep imaging capabilities.

Notes

1. A companion paper accepted for publication in AAS Journals presents further analysis of the quasar pair, the gas bridge between them, and their host galaxies using observations made with the Atacama Large Millimeter/submillimeter Array (ALMA).
2. There were candidates, but they are difficult to separate from possible gravitational lensing images of a single quasar. There are also some candidates to be binary active galactic nuclei embedded in separate reionization-era galaxies, but these have much lower luminosities (Black hole activity) from quasars and are two components within a galaxy that are qualitatively different from what is described here.

Reference: “Discovery of Merging Twin Quasars at z = 6.05” by Yoshiki Matsuoka, Takuma Izumi, Masafusa Onoue, Michael A. Strauss, Kazushi Iwasawa, Nobunari Kashikawa, Masayuki Akiyama, Kentaro Aoki, Junya Arita, Masatoshi Imanishi, Rikako Ishimoto, Toshihiro Kawaguchi, Kotaro Kono, Chien-Hsiu Lee, Toru Nagao, John D. Silverman, and Yoshiki Toba, 5 Apr 2024, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ad35c7

The team consists of Yoshiki Matsuoka (Ehime University, Japan), Takuma Izumi (National Astronomical Observatory of Japan, Tokyo), Masafusa Onoue (Kavli Institute for the Physics and Mathematics of the Universe, Japan), Michael A. Strauss (Princeton UniversityUSA), Kazushi Iwasawa (University of Barcelona, ​​Spain), Nobunari Kashikawa (University of Tokyo, Japan), Masayuki Akiyama (Tohoku University, Japan), Kentaro Aoki (Subaru Telescope, National Astronomical Observatory of Japan, USA), Junya Arita ( University of Tokyo, Japan), Masatoshi Imanishi (National Astronomical Observatory of Japan, Advanced Research Univ [SOKENDAI]Japan), Rikako Ishimoto (University of Tokyo, Japan), Toshihiro Kawaguchi (Onomichi City University, Japan), Kotaro Kohno (University of Tokyo, Japan), Chien-Hsiu Lee (WM Keck Observatory, USA), Toru Nagao (Ehime University , Japan), John D. Silverman (Kavli Institute for the Physics and Mathematics of the Universe, Japan) and Yoshiki Toba (Ehime University, Japan, National Astronomical Observatory of Japan, Tokyo, Institute of Astronomy and Astrophysics Academia Sinica, Taiwan )