Compelling evidence that the structure of matter surrounding supermassive black holes has changed over cosmic time has been uncovered by an international team of astronomers.
If true, the research led by the National Observatory of Athens and published today in Monthly Notices of the Royal Astronomical Society would challenge a fundamental law which has existed for almost five decades.
Quasars – first identified in the 1960s – are some of the brightest objects in the universe. They are powered by supermassive black holes as matter, pulled by strong gravity, spirals inwards, forming a rotating disc-like structure which eventually plunges into the black hole.
This disc is extremely hot because of the friction between matter particles as they revolve around the black hole. It produces 100 to 1,000 times as much light as an entire galaxy containing 100 billion stars, generating a glow that outshines its host galaxy and everything in it. This vast amount of ultraviolet light can be observed by telescopes, allowing astronomers to find quasars at the edge of the universe.
The ultraviolet light of the disc is also believed to be the fuel for the much more energetic X-ray light produced by quasars: the ultraviolet light rays as they travel through space intercept clouds of highly energetic particles very close to the black hole, a structure also known as the “corona”.
As they bounce off these energetic particles, the ultraviolet rays are boosted in energy and generate intense X-ray light that our detectors can also spot.
Because of their shared history, the X-ray and ultraviolet emissions of quasars are tightly connected – brighter ultraviolet light typically means stronger X-ray intensity. This correlation, discovered nearly 50 years ago, provides fundamental insights into the geometry and physical conditions of the material close to supermassive black holes and has been the focus of intense research for decades.
The latest research adds a new twist to previous studies by challenging the universality of the correlation – a fundamental assumption that implies that the structure of matter around black holes is similar throughout the universe.
It shows that when the universe was younger – about half its present age – the correlation between the X-ray and ultraviolet light of quasars was significantly different from that observed in the nearby universe. The discovery suggests that the physical processes linking the accretion disc and the corona around supermassive black holes may have changed over the last 6.5 billions of years of cosmic history.
“Confirming a non-universal X-ray-to-ultraviolet relation with cosmic time is quite surprising and challenges our understanding of how supermassive black holes grow and radiate,” said Dr Antonis Georgakakis, one of the study’s authors.
“We tested the result using different approaches, but it appears to be persistent.”
The study combines new X-ray observations from eROSITA X-ray telescope and archival data from the XMM-Newton X-ray observatory of the European Space Agency to explore the relation between X-ray and ultraviolet light intensity of an unprecedentedly large sample of quasars. The new eROSITA’s wide and uniform X-ray coverage proved decisive, enabling the team to study quasar populations on a scale never before possible.
The universality of the UV-to-X-ray relation underpins certain methods that use quasars as "standard candles" to measure the geometry of the universe and ultimately probe the nature of dark matter and dark energy. This new result highlights the necessity for caution, demonstrating that the assumption of unchanging black hole structure across cosmic time must be rigorously re-examined.
“The key advance here is methodological,” said postdoctoral researcher Maria Chira, of the National Observatory of Athens, who is the paper’s lead author.
“The eROSITA survey is vast but relatively shallow – many quasars are detected with only a few X-ray photons. By combining these data in a robust Bayesian statistical framework, we could uncover subtle trends that would otherwise remain hidden.”
The full set of eROSITA all-sky scans will soon allow astronomers to probe even fainter and more distant quasars. Future analyses using these data – together with next-generation X-ray and multiwavelength surveys – will help reveal whether the observed evolution reflects a genuine physical change or simply selection effects.
Such studies will bring new insight into how supermassive black holes power the most luminous objects in the universe, and how their behaviour has evolved over cosmic time.
ENDS