An extensive review of 1.3 million galaxies alongside 8,000 supermassive black holes emitting X-rays has been conducted to ascertain why these colossal gravitational entities are expanding at a reduced pace compared to historical observations.
Galaxy J033225 is more radiant in X-rays than galaxy J033215 due to its enhanced accretion. (Image credit: X-ray: NASA/CXC/Penn State Univ./Z. Yu et al.; Optical (HST): NASA/ESA/STScI; Infrared: NASA/ESA/CSA/STScI; Image Processing: NASA/CXC/SAO/P. Edmonds, L. Frattare)Subscribe to our newsletter
For an extended period, scientists have been perplexed by the considerably slower accretion rates of the universe’s most massive black holes over the last 10 billion years. A recent investigation now proposes a plausible explanation for this cosmic puzzle: a scarcity of gas.
Supermassive black holes (SMBHs) possess formidable gravitational capabilities that enabled them to attain masses many millions or billions of times that of the sun at remarkably swift velocities during the initial epochs following the Big Bang. Nevertheless, SMBHs have been exhibiting progressively slower expansion since the epoch known as “cosmic noon,” when the cosmos was less than a quarter of its present age.
The prevailing hypothesis suggests that the reduction in growth rates is a consequence of a decreased availability of matter for them to consume, according to a paper released on December 17th in The Astrophysical Journal.
“We were aware that black holes were developing at a slower pace, but the underlying cause remained elusive. It turns out that individual black holes are accreting material at a considerably reduced rate, rather than there being a scarcity of growing or smaller black holes,” commented Fan Zou, a co-author of the study and an astronomer at the University of Michigan, in an email correspondence with Live Science.
Beefy black holes on a diet
The study of black hole growth is fundamental to comprehending galactic evolution and the genesis of stars, given the synchronized development of SMBHs and their host galaxies. The dimensions of SMBHs also show a correlation with the collective mass of stars and their erratic movements within a galaxy’s bulge—the centrally concentrated, football-shaped region packed with stars.
To quantify the changes in black hole growth across cosmic history, the researchers leveraged data from nine extragalactic surveys arranged in a “wedding cake” configuration. These layered observations encompass broad surveys of extensive, relatively proximate celestial regions, as well as highly focused, deep “pencil-beam” explorations of smaller areas. This data was gathered using leading X-ray space observatories, including NASA’s Chandra X-ray Observatory, the European Space Agency’s XMM-Newton, and Germany-Russia’s eROSITA.
“X-ray emissions serve as perhaps the most effective indicator of black hole accretion,” stated Zhibo Yu, the lead author and an astronomer at Penn State, in an email to Live Science. “They are consistently generated by actively growing supermassive black holes and exhibit a strong contrast against the background starlight. Furthermore, their high penetrating capability—similar to that used in medical imaging—makes them less susceptible to interference from obscuring gas and dust within the galaxy.”
In total, the research team analyzed multiwavelength observations encompassing 1.3 million galaxies and 8,000 SMBHs actively accreting matter and emitting X-rays, with the aim of determining the reason for the significant decline in the black holes’ growth velocity.
Have black holes had their heyday?
The investigation explored three primary hypotheses: Are contemporary black holes consuming less matter? Alternatively, are they inherently smaller and thus less gravitationally voracious than their ancient counterparts? Or is the overall population of actively growing black holes smaller?
The researchers concluded that the rate at which black holes are accreting has decelerated due to a reduction in the available cold gas since the era of cosmic noon, approximately 10 billion years ago. “What most astonished me was our ability to pinpoint the principal factor; indeed, there is a dominant reason rather than a convoluted mixture,” explained Zou.
Illustrations depicting various theories regarding the decline in black hole growth rates over cosmic epochs. (Image credit: Penn State/Z. Yu)
This observed reduction in growth rates appears to be substantial. “Our most precise estimation indicates a decrease by a factor of 22,” stated Neil Brandt, an astrophysicist at Penn State and co-author of the study, in an email to Live Science. While this research did not delve into the astonishingly rapid growth of black holes in the very early universe, “it provides the most comprehensive analysis to date for the latter 75% of cosmic time— a significant majority!”
Future investigations may incorporate supplementary datasets, such as wide-field X-ray surveys from Chandra and XMM-Newton, along with multiwavelength data from other astronomical instruments. This will enable astronomers to identify larger populations of SMBHs, including more ancient examples and those obscured by dense clouds of dust and gas.
Ultimately, the research reinforces the conclusion that the era of prodigious SMBH expansion is in the past. “We do not anticipate the emergence and significant growth of numerous SMBHs in the future,” Zou remarked. “In fact, our findings from 2024 suggest that the number of SMBHs had largely stabilized by 7 billion years ago and is likely to remain so going forward.”
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Sourse: www.livescience.com