An international team of astronomers has made an unprecedented discovery by detecting the most distant pair of merging quasars ever observed. Dating back just 900 million years after the Big Bang, these mysterious cosmic objects shed new light on the pivotal era of Reionization.
Galaxies that meet in the immensity of space
Despite the dizzying distances that separate them, galaxies are far from being fixed in the Universe. They regularly collide, merge, or even overlap. And this is precisely what astronomers have spotted with this pair of merging quasars, the most distant ever detected.
These two galactic hearts, where gas and dust are sucked in by a central supermassive black hole, emit a phenomenal amount of light. A typical characteristic of quasars, but this dynamic duo is distinguished by its antiquity: it dates back to the Cosmic Dawn, a little-known and crucial period in the history of the Universe.
When quasars illuminate the Cosmic Dawn
The Reionization Epoch, which extends from approximately 50 million to 1 billion years after the Big Bang, marks a turning point in cosmic evolution. It is at this moment that the first stars and galaxies appear, bathing in light a Universe hitherto plunged in darkness.
For nearly 400 million years, ultraviolet radiation emitted by these cosmic pioneers, including quasars, propagated and ionized the surrounding hydrogen. A fundamental process that ends the dark ages and lays the foundations for the vast, bright structures observable today.
To better understand the exact role of quasars during this pivotal period, astronomers are actively tracking these distant objects. As Yoshiki Matsuoka, co-author of the study, explains, their statistical properties are instructive:
A duo of quasars that stands out
Although around 300 quasars have already been spotted in the Reionization epoch, none formed a merging pair until now. Potential candidates had been proposed, but it was difficult to differentiate them from gravitational lensing effects produced by a single quasar.
It was while reviewing images from the Subaru Telescope that Yoshiki Matsuoka noticed two similar faint red dots, side by side. Intrigued, he led additional observations with the Gemini North and Subaru telescopes to confirm the nature of these curious objects.
The measurements revealed that the quasars were so faint that they eluded even near-infrared detection by the largest terrestrial telescopes. Part of their optical light would actually come from ongoing star formation in their host galaxies.
A cosmic mega-merger under surveillance
Another size indicator: the central black holes of these quasars are behemoths, each weighing about 100 million times the mass of the Sun. This feature, coupled with the presence of a gas bridge connecting them, suggests that they are merging during a galactic mega-merge.
This long-awaited discovery opens new perspectives for probing the epoch of Reionization, a true cosmic bridge between the first structures of the Universe and the complex cosmos that we observe billions of years later. The most distant objects from this period are valuable keys to understanding the dawn of our Universe.
And astronomers are eager to discover more. The Vera C. Rubin Observatory, which will begin its ambitious decade-long LSST survey next year, could spot equally exciting new quasars. Enough to continue to unravel the mysteries of the Cosmic Dawn and the birth of the first lights of the Universe.
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