Black holes swallow everything—including light—which explains why we can’t see them. But we can observe their immediate surroundings and learn about them. And when they’re on a feeding binge, their surroundings become even more luminous and observable.
This increased luminosity allowed astronomers to find a black hole that was feasting on material only 800 million years after the Universe began.
Even with everything astrophysicists have learned, black holes are still mysterious. We know that the largest ones—supermassive black holes (SMBH)—reside in the centers of galaxies like the Milky Way. But the history of their formation, growth, and evolution is still shrouded in cosmic mystery.
Astrophysicists can infer the presence of these monsters in the heart of galaxies by the effect their massive gravitational pull has on nearby stars. But a better opportunity to study them is when they’re actively feeding. An actively feeding black hole is called an active galactic nucleus (AGN,) and when an AGN is extremely luminous, it’s called a quasar. As material swirls around their accretion disks, it heats up and emits x-rays.
Scientists have struggled to locate quasars in the early Universe, but it’s an important goal in black hole research. They need to find them in order to trace their development over time. One stumbling block in their efforts is the time period correlating with redshifts greater than z=6, about 12.716 billion years ago, or about one billion years after the Big Bang.
Now a team of researchers from the Max Planck Institute for Extraterrestrial Physics (MPE) has found an extremely x-ray luminous quasar at redshift z=6.56, only about 800 million years after the Big Bang. They presented their findings in a paper published in the journal Astronomy and Astrophysics. Their paper is “X-ray emission from a rapidly accreting narrow-line Seyfert 1 galaxy at z = 6.56.” The lead author is Julien Wolf, a Ph.D. student in high-energy astrophysics at MPE.
The x-rays from this quasar, named J0921+0007, had to travel a long way through space and time to reach us. The instrument it reached was the eROSITA (extended ROentgen Survey with an Imaging Telescope Array) x-ray instrument on the Spektr-RG space observatory. eROSITA found the quasar in its Final Equatorial-Depth Survey (FEDS.) The Chandra Space Telescope also spotted it.
That survey is important because, currently, astrophysicists know of only 50 quasars with redshift z>5.7, when the Universe was less than one billion years old. By finding more, scientists hope to place a lower limit on black hole accretion well into the Epoch of Re-ionization, when the first stars and galaxies formed.
This quasar is especially interesting because its so bright in x-rays. But it’s also a low-mass black hole with only 250 million solar masses. Most high redshift galaxies like this one host black holes with between one to ten billion solar masses. For this one to be detected, it must be accreting matter at a very high rate and it must be growing rapidly. That’s the only explanation for its brightness in x-rays.
“We did not expect to find such a low-mass AGN already in our very first mini-survey with eROSITA”, said lead author Wolf, who searches for the most distant supermassive black holes in eROSITA data as part of his Ph.D. “It is the most distant serendipitous X-ray detection to date and its properties are rather atypical for quasars at such high redshifts: it is intrinsically faint in visible light but very luminous in X-rays.”
This quasar is similar to a type of galaxy called narrow-line Seyfert-1 galaxies. They’re a type of active galaxy in the local Universe. They’re associated with SMBHs with less than 100 million solar masses that are accreting matter at a high rate. They could be younger than their higher-mass SMBH counterparts.
What does it mean to find this quasar this early in the Universe? It sheds light on the earliest stages of black hole formation.
It takes an extraordinarily high concentration of mass to form a black hole. In the modern Universe, those densities are found only in stars. But in the early Universe, before so much expansion, there were other densities. Somehow, they may have collapsed into black holes, and the only reason that entire Universe didn’t collapse into one is that expansion overpowered it.
Understanding the density fluctuations in the early Universe that allowed black holes to form is part of the cutting edge in astrophysics and cosmology. So while this single detection of an actively feeding and rapidly growing black hole in the Epoch of Reionization won’t answer all of our questions, it’s a piece of the puzzle.
How black holes formed in the early Universe is only one question. Another question is how did they grow? One way astrophysicists try to track black hole growth is by tracing their accretion through cosmic time via the X-ray Luminosity Function (XLF.) XLF is associated with accretion and there are varying models explaining the association. Detecting these ancient quasars in x-rays helps place constraints on the XLF and will help astrophysicists clarify these models.
“At z = 6.56, J0921+0007 is the most distant X-ray-selected AGN to date and can therefore be used to impose constraints on the high-z XLF,” the authors point out in their paper.
The Eddington limit also plays a role in this work. The Eddington limit is the maximum luminosity that an object can achieve when outward radiation and inward gravitation are balanced. Astrophysicists think that the Universe’s earliest black holes can exceed this limit because conditions are right for rapid accretion. To find out more about these super-Eddington black holes and the overall black hole accretion density in the early Universe, researchers need to find more of them. “In order to quantify how much of the accretion density is in fact driven by young, super-Eddington black holes, a wider survey area will be required at this depth to obtain a more informative sample. This will be made possible in the cumulative eROSITA All-Sky Survey,” the authors write in their conclusion.
This ancient black hole isn’t the only piece of the puzzle found by eROSITA and its Final Equatorial-Depth Survey. The survey has already found five more of them. The MPE research team will present those findings in a future paper. Based on all of these detections, the scientists expect to find hundreds more of them with the survey.
Super-massive black holes are dominant objects in the Universe. How they formed, how they grew so large, and how they became symbiotic with the growth of huge galaxies are all unanswered questions.
But this work shows researchers are making progress.