TOP SCEINCE
Pair of merging quasars at cosmic dawn
Since the very first instant after the Big Bang the Universe has been expanding. This means that the early Universe was considerably smaller and early-formed galaxies were more likely to interact and merge. Galaxy mergers fuel the formation of quasars — extremely luminous galactic cores where gas and dust falling into a central supermassive black hole emit enormous amounts of light. So when looking 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 mergers. However, they have been surprised to find exactly none — until now.
Cosmic Dawn spanned from about 50 million years to one billion years after the Big Bang. During this period the first stars and galaxies began appearing, filling the dark Universe with light for the first time. The arrival of the first stars and galaxies kicked off a new era in the formation of the cosmos known as the Epoch of Reionization.
The Epoch of Reionization, which took place within 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 throughout the cosmos, interacting with the intergalactic medium and stripping the Universe’s primordial hydrogen atoms of their electrons in a process known as ionization. The Epoch of Reionization was a critical epoch in the history of the Universe that marked the end of the cosmic dark ages and seeded the large structures we observe in our local Universe today.
To understand the exact role that quasars played during the Epoch of Reionization, astronomers are interested in finding and studying quasars populating this early and distant era. “The statistical properties of quasars in the Epoch of Reionization tell us many things, such as the progress and origin of the reionization, the formation of supermassive black holes during Cosmic Dawn, and the earliest evolution of the quasar host galaxies,” said Yoshiki Matsuoka, an astronomer at Ehime University in Japan and lead author of the paper describing these results, published in the Astrophysical Journal Letters.
About 300 quasars have been discovered in the Epoch of Reionization, but none of them have been found in a pair. That is until Matsuoka and their team were reviewing images taken with the Hyper Suprime-Cam on the Subaru Telescope and a faint patch of red caught their eye. “While screening images of quasar candidates I noticed two similarly and extremely red sources next to each other,” said Matsuoka. “The discovery was purely serendipitous.”
The team was not sure that they were a quasar pair since 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 Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope and the Gemini Near-Infrared Spectrograph (GNIRS) on Gemini North. The spectra, which break down the emitted light from a source into its component wavelengths, obtained with GNIRS were crucial to characterizing the nature of the quasar pair and their host galaxies.
“What we learned from the GNIRS observations was that the quasars are too faint to detect in near-infrared, even with one of the largest telescopes on the ground,” said Matsuoka. This allowed the team to estimate that a portion of the light detected in the optical wavelength range is not coming from the quasars themselves, but from ongoing star formation taking place in their host galaxies. The team also found that the two black holes are whoppers, each being 100 million times the mass of the Sun. This, coupled with the presence of a bridge of gas stretching between the two quasars, suggests that they and their host galaxies are undergoing a major-scale merger [1].
“The existence of merging quasars in the Epoch of Reionization has been anticipated for a long time. It has now been confirmed for the first time,” said Matsuoka [2].
The Epoch of Reionization connects the earliest formation of cosmic structure to the complex Universe that 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 NSF-DOE Vera C. Rubin Observatory’s decade-long Legacy Survey of Space and Time (LSST), beginning in 2025, which is poised to detect 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 taken with the Atacama Large Millimeter/submillimeter Array (ALMA).
[2] There have been candidates, but it is difficult to separate them from possibly gravitationally-lensed images of a single quasar. There are also some candidates for being dual active galactic nuclei embedded in individual Epoch of Reionization galaxies, but these have much lower luminosity (black hole activity) than quasars and are two components within a single galaxy, which are qualitatively different from what is described here.