Since the first moment of its existence, 13.76 billion years ago, the Universe has been expanding. Which implies that at the beginning of its existence it was much smaller than what we can see today and that, with less space available, … The first galaxies were much more likely than today’s galaxies to interact, to collide with each other and merge.
And it is precisely that, the merger of galaxies, that drives the formation of one of the most fascinating objects that can be seen in the sky: quasars, extremely luminous galactic nuclei in which the matter that falls into the central supermassive black hole It emits huge ‘jets’ of light. Bright and extremely energetic jets that can extend even hundreds of thousands of light years away from the galaxy that emits them.
Logically, and given the large number of galaxies merging and forming quasars in a primitive and ‘small’ Universe, when looking back astronomers expected to also find many quasars so close to each other as to form ‘pairs’, and even merge. . If two galaxies that join together give rise to a quasar, one of the brightest objects in the Universe, what will the merger of two quasars be like? However, to general confusion, one of these mergers has never been observed. Until now.
In this image, taken with the Hyper Suprime Cam of the Subaru telescope, two quasars in the process of merging are observed for the first time in Cosmic Dawn
NOIRLab/NSF/AURA/TA Rector (University of Alaska Anchorage/NSF NOIRLab), D. de Martin (NSF NOIRLab) & M. Zamani (NSF NOIRLab)
The first merger of two quasars
To see it, a Japanese team of astronomers, led by Yoshiki Matsuoka, from Matsuyama University, had to add the power of two large telescopes, both at the Mauna Kea Observatory, in Hawaii: the Gemini North (which is half of the Gemini International Observatory, whose other telescope, Gemini South, is in Chile) and the Japanese Subaru. In this way, they managed to observe, for the first time, a pair of quasars merging in the early Universe, ‘only’ 900 million years from the Big Bang, that is, almost 13,000 million light years away from Earth. The work has just been published in ‘Astrophysical Journal Letters’.
They are the first confirmed pair of quasars in the period in the history of the Universe known as ‘Cosmic Dawn’, which extended from about 50 million years to one billion years after the Big Bang and during which the first ones began to appear. stars and galaxies, that is, the first lights in the midst of the previous darkness.
The arrival of those first stars and galaxies began a new era in the formation of the cosmos known as the ‘Epoch of Reionization’, a transitional stage that took place approximately 400 million years after the Big Bang and during which ultraviolet light of the first stars, galaxies and quasars spread throughout the cosmos, interacting with the intergalactic medium and stripping primordial hydrogen atoms of their electrons. The Epoch of Reionization was critical in the history of the Universe, marking the end of the dark ages and sowing the seeds of the great structures we observe today in our local Universe.
To understand the exact role quasars played during the Reionization Epoch, astronomers have long struggled to find and study the quasars that populated this early, distant era. “The statistical properties of quasars in the Epoch of Reionization – explains Matsuoka – 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 galaxies. “hosts of quasars.”
Fortuitous discovery
So far, about 300 quasars have been discovered in the Reionization Epoch, but none of them were part of a pair. Until Matsuoka and his team, during a review of images taken by the Subaru Telescope’s Hyper Suprime Cam, came across a faint reddish spot that caught their attention.
“While examining images of quasar candidates,” Matsuoka recalls, “I noticed two similar, extremely red sources, side by side. The discovery was purely fortuitous.
At first, researchers weren’t sure what they were seeing was actually a pair of quasars, since signals from distant quasar candidates are often ‘contaminated’ by many other light sources, such as foreground stars and galaxies. , or by the distortion effects of gravitational lensing. Therefore, to confirm the true nature of these objects, the team performed follow-up spectroscopy using both the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope and the Gemini Near-Infrared Spectrograph (GNIRS) on the Gemini North Telescope. . The spectra obtained with GNIRS, which decompose the light emitted by a source into its component wavelengths, were crucial in characterizing the nature of the quasar pair and their host galaxies.
“What we learned from the GNIRS observations – continues the researcher – was that quasars are too faint to detect in the near infrared, even with one of the largest telescopes on Earth.” Which allowed the team to estimate that some of the light detected in the visible wavelength range did not come from the quasars themselves, but from the ongoing star formation process taking place in the host galaxies.
Matsuoka and his colleagues also discovered that the two black holes at the center of both galaxies are really big, each about 100 million times the mass of the Sun. This, along with the presence of a ‘bridge’ of gas that stretches between the two quasars, suggests that they and their host galaxies are right in the middle of a large-scale merger process.
«The existence of merged quasars in the Reionization Epoch – Matsuoka said – has been assumed for a long time. But now, for the first time, it has been confirmed.
