New Research Reveals Slowdown in Black Hole Growth, Offering Insights Into Astrophysical Phenomena

Astrophysicists conducting a detailed study across the universe's long-standing 13.8 billion-year history have discovered compelling evidence that indicates a deceleration in the growth rate of black holes. These extraordinary cosmic entities possess an exceptionally powerful gravitational force capable of confining even light within their grasp. Among them, "supermassive" black holes particularly capture attention, impressively weighing millions to billions of times the mass of our Sun.

Typically residing at the hearts of galaxies, these gargantuan black holes even find their abode within our own Milky Way galaxy. The question of how these supermassive black holes attain such colossal proportions prompted our team of astrophysicists to embark on a journey through time, unraveling the growth trajectory and mechanisms behind these cosmic giants.

By meticulously constructing a model to encapsulate the comprehensive growth history spanning an immense expanse of 12 billion years, researchers proposed that the growth of supermassive black holes primarily occurs through two distinctive avenues. The first involves accretion, a process wherein these black holes consume gas from their host galaxies. The second mechanism involves the merging of these cosmic entities when galaxies collide.

When supermassive black holes undergo the accretion process, they emit remarkably powerful X-rays, a form of high-energy light that lies beyond the range of human sight. Similar to X-rays employed during dental examinations, albeit with lower energy levels, these emissions stem not from the black holes themselves, but from the gas surrounding their outer vicinity. As gas succumbs to the gravitational pull of a black hole, it becomes highly heated and releases light, particularly X-rays. Consequently, an increase in gas intake by supermassive black holes correlates with heightened X-ray production.

The advent of advanced X-ray facilities, such as Chandra, XMM-Newton, and eROSITA, equipped with exceptional capabilities for capturing X-rays emitted by numerous accreting supermassive black holes across the universe, has greatly facilitated this research. Insights derived from extensive data collected over two decades from these groundbreaking facilities enable astronomers to precisely estimate the growth rate of supermassive black holes through gas consumption. Remarkably, on average, these celestial phenomena ingest sufficient gas each year to bulk up as much as our Sun's mass, although precise valuation relies on a variety of factors.

Exceptionally illuminating, the obtained data highlights a direct correlation between the growth rate of a black hole, averaged over millions of years, and the total stellar mass inherent within its host galaxy. This nexus further bolsters our understanding of the interplay between black holes and their galactic environments.

Aside from accretion, the growth of supermassive black holes can also occur through mergers, whereupon two black holes combine forces to form a larger, more massive black hole when galaxies experience collisions. Comprehensive cosmological simulations employing supercomputers have succeeded in forecasting the frequency of these mergers, enabling scientists to delineate the evolving cosmos. Similar to countless individual bricks contributing to the construction of a vast structure, galaxies along with their embedded supermassive black holes undergo multiple mergers throughout cosmic history.

Harnessing the potential of X-ray observations and supercomputer simulations, our research team ingeniously intertwines and analyzes the two growth channels of gas consumption and mergers. Thus, a comprehensive growth history is ascertained, meticulously mapping the expansion of black holes throughout the universe over billions of years.

Consequently, this ambitious endeavor yielded a fascinating revelation—supermassive black holes experienced significantly accelerated growth during the universe's earlier stages when copious amounts of gas were available. Yet, as the universe evolved, these black holes saw a considerable deceleration in growth due to the gradual depletion of gas reserves. Approximately 8 billion years ago, the number of supermassive black holes stabilized, experiencing only minimal further amplification.

It is noteworthy that when gas availability becomes scarce for supermassive black holes to expand through accretion, mergers represent an alternative means for their continued growth. Intriguingly, our growth history portrays instances of mergers rather infrequently. On average, the most massive black holes assimilate mass via mergers at a rate equivalent to the Sun's mass every few decades.

While this research has shed light on the accumulation of over 90% of current black hole mass throughout the past 12 billion years, investigation regarding early universe growth of black holes continues to captivate scientific interest. Unveiling insights into the remaining fraction of black hole mass still poses a compelling challenge in our pursuit to better comprehend the universe's mysterious beginnings. As the astronomical community embarks on this collective exploration, we anticipate exciting breakthroughs in our understanding of early, supermassive black holes.