Chandra Reveals an Extremely Fast Accreting Black Hole in the Early Universe
Luca Ighina, postdoctoral fellow at the Center for Astrophysics | Harvard and Smithsonian
We are happy to welcome Luca Ighina as a guest blogger. Luca is a postdoctoral fellow at the Center for Astrophysics | Harvard and Smithsonian and the first author of a paper that is the subject of our latest Chandra press release. In his research he studies the most distant supermassive black holes with a focus on those able to expel part of the material they are pulling inwards in the form of jets moving at close to the speed of light. To this end he uses the most sensitive telescopes around the world covering the entire electromagnetic spectrum, from the radio to the optical-near-infrared to the X-ray band. He received his PhD from the Insubria University (Como, Italy), where he mainly worked at the INAF institute in Milano-Brera (Italy), along with a one-year visit to the ICRAR institute for radio astronomy in Perth (Western Australia).
How do supermassive black holes grow in the early Universe?
Most galaxies observed in the Universe host a supermassive black hole at their center. Some of them, called active galactic nuclei (AGN) or quasars, grow in mass as surrounding material falls inwards through a process called accretion. Astronomers have found some extreme examples of quasars that can reach masses of several billion times that of the Sun, only one billion years after the Big Bang. Understanding how these black holes formed and grew so rapidly remains one of the major open questions in modern astrophysics.
Several theories have been put forward in the last few years, proposing different explanations for this rapid growth. Typically these models imply that the massive black holes we observe formed already massive – almost one million times the mass of our Sun – or they accrete much more material than previously thought. However, observational evidence for either scenario is still scarce since black holes in the early Universe, namely one billion years after the Big Bang, are extremely rare and faint.
Material falling onto a black hole follows a spiral orbit, producing what is normally called an accretion disk. As the infalling gas loses energy it heats up due to friction and produces radiation. The light emitted by the accretion disc tends to push out the gas, and, if the radiation is extremely strong, can actually stop the accretion flow. The more material falling in, the more radiation is produced. Therefore, there is a theoretical limit when the forces from gravity and radiation are balanced out and the accretion/growth of the black hole is maximized. This limit is referred to as the ‘Eddington limit’, after Sir Arthur Stanley Eddington, a British astronomer who first came up with this calculation.
Scientists have proposed scenarios in recent theoretical work where the accretion of a black hole actually exceeds the Eddington limit by considering general relativity effects and a much more complex geometry. The resulting emission in this ‘super-Eddington’ state is quite different from the typical one of a quasar, especially in the X-rays, where we would expect a very soft X-ray emission (i.e., most of the X-ray flux is produced at low energies.
RACS J0320-35, a unique laboratory to study fast black hole growth
In a paper that we recently published, we reported the multi-wavelength analysis of a quasar we observe at a distance of about 12.8 billion light-years from Earth. (For those that follow astronomy, this translates into a redshift, or “z”, of about 6.13.) The existence of this quasar, named RACS J0320-35, supports the scenarios of very fast growth during the first phases of black hole evolution. We discovered RACS J0320-35 more than two years ago (see Ighina et al. 2023) and based on the strong radio emission produced by the source we could infer the presence of two powerful jets of relativistic particles that are ejecting part of the accreting material into the surrounding gas. As a reference, only about 10% of the total quasar population is able to produce such powerful relativistic jets. Nevertheless, the properties of its accretion disk based on publicly available optical and infrared data did not show any difference from the bulk of the quasar population.
Only after performing dedicated observations with the most sensitive X-ray telescope currently available, the Chandra X-ray Observatory, did we discover that the quasar has very strong X-ray emission. It may be the brightest ever seen in X-rays at z > 6, suggesting the presence of large amounts of material accreting onto the black hole. Moreover, most of this X-ray emission is soft, fully consistent with super-Eddington accretion models. Indeed, this is also the quasar with the ‘softest’ X-ray emission currently known in the high redshift Universe.
This discovery represents a significant step forward in our understanding of black hole growth at high redshifts. Indeed, RACS J0320−35 represents a unique laboratory where we can put super-Eddington accretion models to the test. To this end further observations at different epochs and at different wavelengths, such as in the near-infrared with NASA’s JWST, will be crucial. Finally, the fact that RACS J0320−35 also hosts powerful relativistic jets could imply that they are linked to the fast accretion of the central black hole. This hypothesis needs further testing, but if confirmed it would have a huge impact on our understanding of black hole formation and evolution in the early Universe.