Observations of ancient galaxies called "Little Red Dots" by the James Webb Space Telescope (JWST) could finally answer the question: which comes first, the black hole or its galaxy? It turns out that the answer isn't what scientists expected and could thus represent a complete paradigm shift in our understanding of how black holes grow.
Little Red Dots were first spotted in 2022 by the JWST, immediately presenting themselves to astronomers as something completely new, perhaps a type of galaxy never seen before. The mystery of these objects deepened when scientists discovered that they are remarkably common in the infant universe but seem to disappear around 1.5 billion years after the Big Bang. But Little Red Dots are far from the only cosmic mystery that the JWST has dropped into the lap of scientists.
The $10 billion space telescope has also discovered a wealth of supermassive black holes with masses millions to billions of times that of the sun prior to the universe being 1 billion years old. That is problematic because the feeding and merging processes that allow black holes to grow to supermassive status had always been thought to take longer than 1 billion years.
This new study of Little Red Dots by the JWST indicates that maybe supermassive black holes were born directly without needing a massive star to live for millions of years before collapsing to birth a stellar-mass black hole. It also means that these early supermassive black holes would not need to gorge on copious amounts of gas and dust from their host galaxies to grow. That means these black holes could form before the galaxies that will eventually host them come together.
"This is a remarkable finding," team member Roberto Maiolino of the University of Cambridge in the United Kingdom, said in a statement. "It's a paradigm shift, a total revisiting of the classical scenarios of how black holes form and grow." The team's research was published on Wednesday (May 27) in the journals Nature and the Monthly Notices of the Royal Astronomical Society
Little Red Dots put black holes on the spot with help from Einstein
To reach their conclusion, scientists focused on the Little Red Dot designated Abell2744-QSO1 (QSO1), which existed 700 million years after the Big Bang. This means that the light from this ancient galaxy, which is just 1,300 light-years wide, has been travelling to Earth for just over 13 billion years.
QSO1 is easier to study than other Little Red Dots because of a phenomenon called gravitational lensing.
First suggested by Einstein in 1915, gravitational lensing occurs when an object of great mass sits between a more distant background object and Earth. As light passes this middle or "lensing" object, its path is curved by the warp in spacetime the lensing body causes; the closer to the object the light passes, the more curved its path is. This means light from the background objects can arrive at our telescopes at different times, thus magnifying the background object.
In the case of QSO1, this Little Red Dot is being gravitationally lensed by the galaxy cluster Abell 2744, also known as Pandora's Cluster.
Researchers had initially thought that QSO1 is actually just a supermassive black hole with a mass 40 million times greater than the sun, surrounded by a cloud of hydrogen and helium gas. However, scientists couldn't be entirely sure about the mass of this black hole.
"Before now, all of the mass measurements of black holes in the early universe have been indirect, based on assumptions from what we know about them in the local universe," team member Francesco D'Eugenio, also of the University of Cambridge, said. "We didn't know if those assumptions really apply to the distant universe."
This team reasoned that if the black hole heart of QSO1 is as massive as initially thought, then its mass should be observable in the motion of the gas swirling around it. They therefore used the JWST's NIRSpec (Near Infrared Spectrograph) instrument to map the motion of this gas, finding it orbits a central point similar to how the planets of the solar system orbit the sun, a phenomenon called Keplerian motion.
"This is important because it tells us that most of the mass of QSO1 is concentrated in the black hole at the center," team co-leader Ignas Juodžbalis of Cambridge University said. "If the mass were more distributed, as it would be if there were a lot of stars, the gas would not have this perfect Keplerian rotation."
This allowed the team to directly measure the mass of QSO1's central black hole for the first time.
"This is a phenomenal result," Maiolino added. "It is the first direct measurement of a black hole mass within the first billion years after the Big Bang, and it is consistent with the previous measurements."
This revealed that at 50 million solar masses, the supermassive black hole accounts for an incredible 66% of the total mass of this Little Red Dot. That is a ratio that is thousands of times greater than the ratio of supermassive black hole mass to galaxy mass found in the local universe.
That ratio indicates that this black hole can't have been born from a collapsing star and gradual feeding from the surrounding galaxy, indicating it was born "big" and now has what will eventually grow to be a galaxy taking shape around it.
There are still mysteries to solve surrounding the black hole of QSO1, particularly questions of how it formed. The team thinks that the black hole could have grown from a "heavy seed" born from a collapsing cloud of gas and dust. Or alternatively, it could have been birthed directly during the initial moments of the Big Bang through an as-yet unknown process
What the team is relatively sure of is that QSO1 cannot be rare among Little Red Dots in the early universe. They are now assessing other Little Red Dots to determine if these also harbor supermassive black holes with galaxies in the process of forming around them.
