Supermassive black holes dominate the centers of massive galaxies and regulate their evolution through sustained accretion and energetic feedback. Observations over the last two decades established that the global growth rate of these objects peaked roughly 10 billion years ago and declined steadily afterward. However, astronomers could not determine whether this decline resulted from lower accretion efficiency, reduced black hole masses, or a shrinking population of actively accreting systems.

A new large statistical study based on X-ray observations from NASA’s Chandra X-ray Observatory, together with XMM-Newton and eROSITA, now clarifies the physical origin of this long-recognized trend. By analyzing nearly 1.3 million galaxies and about 8,000 actively growing supermassive black holes, researchers show that the dominant cause of the decline is a steady reduction in accretion rates over cosmic time.

Tracing the history of black hole growth across cosmic time

Supermassive black holes increase their mass primarily through accretion of surrounding gas. As gas falls inward, it forms a rotating disk that converts gravitational energy into radiation. This process produces strong emissions across the electromagnetic spectrum, especially in X-rays. Astronomers therefore rely heavily on X-ray observations to measure black hole growth across large cosmic distances.

However, researchers cannot observe the long-term evolution of a single black hole. Instead, they reconstruct growth histories by studying large populations of galaxies at different distances. Because light from distant objects left them billions of years ago, observations at increasing distance correspond to earlier cosmic epochs. By combining measurements from many systems, astronomers can follow how the average growth of black holes changed across nearly the entire history of the universe.

The present study applies this approach at an unprecedented scale. It combines observations from three major X-ray observatories and builds a dataset that spans both nearby galaxies and very distant systems. As a result, the analysis captures the rise and decline of black hole accretion activity across billions of years with strong statistical confidence.

Building a large and reliable census of growing black holes

The strength of this investigation comes from the size and structure of its dataset. The research team assembled measurements from deep and wide X-ray surveys carried out by Chandra, XMM-Newton, and eROSITA. Each telescope contributed a different observational advantage. Chandra provided extremely deep imaging over small sky regions and revealed faint accreting black holes at large distances. XMM-Newton extended the survey area while maintaining strong sensitivity. Meanwhile, eROSITA mapped wide regions of the sky and detected thousands of additional active galactic nuclei.

These observations formed a layered survey strategy that allowed researchers to probe both rare luminous systems and more common faint sources. This combination ensured that the final dataset captured the full range of black hole activity across cosmic time rather than focusing on only the brightest objects.

The survey ultimately included approximately 1.3 million galaxies. Among them, nearly 8,000 actively accreting supermassive black holes have been detected through their X-ray emission. Such a large sample allowed astronomers to evaluate long-term trends in black hole growth with far greater precision than earlier studies.

Separating the possible causes of the decline in growth

Previous observations showed that the total growth rate of supermassive black holes reached a maximum about 10 billion years ago. Astronomers refer to this period as cosmic noon because both star formation and black hole activity peaked at that time. Afterward, both processes declined steadily.

Nevertheless, researchers could not determine which physical mechanism caused the slowdown in black hole growth. Three possibilities remained under consideration. First, black holes might have begun accreting gas more slowly with time. Second, the typical masses of actively growing black holes might have changed. Third, fewer galaxies might have hosted actively accreting black holes in the later universe.

The new study addresses this question by measuring all three effects within a single consistent framework. The researchers estimated how rapidly black holes accreted matter, how massive those black holes were, and how frequently active accretion occurred among galaxies at different epochs. Because the dataset covers a very large number of systems, the results provide a comparison between these competing explanations.

The analysis shows that declining accretion rates represent the dominant cause of the observed slowdown. Changes in black hole mass contributed a smaller secondary effect, while variations in the fraction of active galaxies played a more limited role.

The role of x-ray observations in revealing hidden activity

X-ray observations played a critical role in reaching this conclusion. Many actively growing black holes lie behind thick layers of gas and dust that block optical light. However, X-ray radiation can escape from these dense regions more easily. As a result, X-ray surveys provide one of the most reliable methods for identifying accreting black holes across cosmic distances.

Chandra contributed deep observations that revealed faint and distant active galactic nuclei. XMM-Newton extended these measurements across wider areas of the sky. Meanwhile, eROSITA provided an all-sky survey that increased the number of known accreting systems substantially. Together, these datasets created a comprehensive map of black hole activity across the observable universe.

Because the study combined both deep and wide surveys, it avoided the biases that can affect smaller observational samples. This allowed researchers to identify the dominant cause of declining black hole growth with strong statistical confidence.

Clear skies!

---