The earliest indication of hefty black hole “seeds” in the early universe may have been found by astronomers.
The existence of some supermassive black holes with masses millions or even billions of times greater than that of the sun could be attributed to the so-called seeds, which grew swiftly enough to exist less than 1 billion years after the Big Bang.
Heavy black hole seeds could be black holes with masses that are roughly 40 million times greater than those of our sun. As opposed to ordinary black holes, which are created when a large star reaches the end of its life and collapses under its own gravity, they are thought to form from the immediate collapse of a massive cloud of gas. Outsize Black Hole Galaxies (OBGs) are galaxies thought to hold such massive black hole seeds.
These galaxies are most likely very far away because they can be viewed with our telescope as they were when our universe, which is 13.8 billion years old, was just 400 million years old. And perhaps now, one of these OBGs has been finally identified by scientists.
The team, coordinated by Akos Bogdán of the Center for Astrophysics at Harvard and the Smithsonian, made the discovery while observing a quasar using the James Webb Space Telescope (JWST) and NASA’s Chandra X-ray Observatory. Hearts of galaxies known as quasars are very bright, energetic objects that are driven by supermassive black holes. In fact, they can be so bright that they can outshine the combined light of all the stars in the galaxy where they are located.
The one Bogdán and his colleagues are studying resides in a galaxy called UHZ1.
And it turned out that UHZ1 data from the JWST and Chandra were in line with what may be predicted from an OBG. By using Chandra, the scientists discovered X-ray emissions. These emissions suggested a feeding or “accreting” black hole connected to the quasar, which was particularly convincing in identifying the nearby galaxy as an OBG.
The researchers found that there was a strong match between their observations and models of the quick growth of heavy black hole seeds. The 10,000 solar mass seed growing over a period of many hundred million years provided the best fit they could find during this comparison.
The first detected OBG candidate, subject to spectroscopic confirmation of its redshift, is what the authors suggest based on the excellent agreement between the observed multi-wavelength properties of UHZ1 and theoretical model template predictions. As the first OBG candidate, UHZ1 thus offers convincing proof that heavy initial seeds were formed through direct collapse in the early universe.
How heavy seeds give black holes a growth boost
Scientists aren’t overly concerned about supermassive black holes’ enormous size. This is due to the fact that these cosmic giants have had billions of years to grow by consuming the gas and dust in their immediate environment and merging with other black holes. For example, Sagittarius A* (Sgr A*), the star at the center of the Milky Way, has enough time to expand to a mass of roughly 4.5 million times that of the sun. The black hole at the center of the galaxy M87 has grown even larger, now weighing almost 5 billion times as much as our star.
However, it is difficult to find such supermassive black holes that existed between just 500 million years and a mere billion years after the Big Bang because these development mechanisms are thought to take place over a period of billions of years. The time required for those mass-gathering techniques to produce such enormous black holes would not have been available. But that’s precisely what scientists using the JWST and other equipment to probe the early universe have discovered.
It’s comparable to observing a family going down the street with two six-foot teenagers and a six-foot toddler in tow. That is a slight issue; how did the child grow to be so tall? According to John Reagan, a Maynooth University research fellow who was not involved in this study. The same holds true for supermassive black holes throughout the cosmos. How did they become so big so fast?
According to one idea, these black holes grew from tiny black hole “seeds,” which gave them an advantage over other black holes in the process of mass accretion.
In this sense, there are primarily two schools of thinking. One the one hand, astronomers speculate that light black hole seeds, which have masses between 10 and 100 times that of the sun, may have evolved into supermassive black holes. The death and collapse of the first generation of stars in the universe, which is the typical process of stellar-mass black hole formation, would theoretically give rise to those light seeds.
On the other side, large seed black holes with enormous masses roughly 100,000 times the mass of the sun may have evolved into early supermassive black holes. These would have originated without going through the “star stage” that other black holes go through, immediately from the collapse of large clouds of stuff. Direct collapse black holes (DCBHs) are the name given to these black holes by astronomers.
Galactic mergers, which were frequent in the early universe and brought supplies of gas and dust for these voids to feast upon, would also present growth opportunities for these DCBHs. Then, it’s possible that more black holes eventually merged and collided with them.
Regan makes the analogy of a toddler who is six feet tall yet is born with a length of three feet. It still seems a little strange (and maybe even a little unsettling), but it does a better job of explaining how the toddler grew to be the size of an adult so quickly than it would have if it had begun with the length of a typical infant.
The finding of UHZ1 as such a galaxy thus supports the presence of heavy black hole seeds and adds credence to their function in early supermassive black hole growth. Other smaller black hole seeds aren’t predicted to give rise to OBGs.
However, the authors explicitly draw attention to the limits of their study and caution against extrapolating that the black hole within UHZ1 grew to supermassive status. They also understand that the likelihood of such growth would be greatly influenced by the environment in which a prospective seed finds itself, with enough of gas and dust being required to sustain its growth.
Even while there is still a lot of work to be done before a population of heavy seed black holes can be verified and their relationship to supermassive black holes in the young universe can be established, these results are at least a start in the right direction.
The researchers added, “We hope to evaluate those sources, investigate their X-ray equivalents with Chandra, and acquire a deeper knowledge of OBGs and heavy seeding physics as JWST identifies additional [far and early] accreting black holes in the coming cycles.
Regan told Space.com that “this detection provides yet more evidence for the heavy seed scenario.” The weight of the evidence is now firmly pointing towards a heavy seed scenario for supermassive black hole formation, in my opinion, when combined with other JWST black hole masses that have been observed.
The team’s work has been published on the paper repository and submitted to the Astrophysical Journal Letters arXiv.