Merging quasars at the Cosmic Dawn
Quasars are extremely bright, active, young galaxies in the distant universe. Astronomers said on June 17, 2024, they’ve spotted two merging quasars as far back as a mere 900 million years after the Big Bang. This puts these merging quasars in a time period known as the Cosmic Dawn, when the first stars and galaxies were beginning to light the universe.
So these quasars hold the titles for most distant pair of merging quasars ever found and the first merging pair seen at the Cosmic Dawn. The scientists published their study in the peer-reviewed journal The Astrophysical Journal Letters on June 17, 2024. A companion paper accepted by the AAS Journal is also available on arXiv.
In the beginning
As far as we know, the universe began with a Big Bang some 13.8 billion years ago. The universe expanded after the Big Bang, so in the early years, objects were much closer together. For that reason, astronomers expected to see quasars close together as galaxies merged. But this is the first time astronomers have spotted merging quasars in the early universe.
This time period – the Cosmic Dawn – lasted from about 50 million years to 1 billion years after the Big Bang. Astronomers say this is the time period when stars began to ignite and the universe went from a dark place to one sprinkled with light. As stars and galaxies grew, their ultraviolet light spread through the universe. As it did, it interacted with the intergalactic medium, or the diffuse gas of atoms and plasma between galaxies. The ultraviolet light’s effect on this gas was to strip atoms of their electrons, in what’s called ionization. Thus, about 400 million years after the Big Bang, we get the Epoch of Reionization.
Epoch of Reionization
During the Epoch of Reionization, the universe got the seeds it needed to create the large-scale structures of visible matter we see today. Previously, astronomers knew of about 300 quasars from the Epoch of Reionization. But the discovery of merging quasars from this time period is a first. And it allows astronomers to better understand what was happening in our universe then. Lead author Yoshiki Matsuoka of Ehime University in Japan said:
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.
The discovery of merging quasars
The scientists found the pair of quasars while reviewing images from the Subaru Telescope, located in Hawaii. Matsuoka said:
While screening images of quasar candidates I noticed two similarly and extremely red sources next to each other. The discovery was purely serendipitous.
When astronomers look back at the early universe, the light from these objects has been stretched due to the expanding universe. So these objects’ light is shifted to the red end of the spectrum. Therefore, seeing a reddish object often means it existed far back in time and has traveled through a greater span of the universe.
The astronomers needed to be sure what they were seeing were two separate objects and not one object gravitationally lensed. Sometimes the light from a closer, massive object can cause the light behind to bend and appear on more than one location of the sky.
Follow-up observations
So, to be sure the objects they saw were merging quasars and not foreground stars or something else, the astronomers conducted follow-up observations. They used the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope and the Gemini Near-Infrared Spectrograph (GNIRS) on Gemini North.
What they found were two enormous black holes powering the quasars. Each black hole has about 100 million times the mass of the sun. Plus, a bridge of gas stretches between the two quasars, confirming the notion that they’re a pair undergoing a merger.
Matsuoka said:
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.
Bottom line: For the first time, scientists have discovered two merging quasars at the Cosmic Dawn, some 900 million years after the Big Bang. These are the most distant merging quasars yet seen.
Source: Discovery of Merging Twin Quasars at z = 6.05
Source: Merging gas-rich galaxies that harbor low-luminosity twin quasars at z = 6.05: a promising progenitor of the most luminous quasars
Via NOIRLab