The story of galaxies’ black hearts has been told almost in full for the first time, as astronomers combined X-ray observations with detailed supercomputer models to trace the growth of supermassive black holes more than 12 billion years of cosmic history.
In doing so, scientists have shown that the black hole at the heart of our planet Milky Way reached its four million solar masses relatively late in its history.
Supermassive black holes are millions of times more massive than our sun up to billions of times more massive, but their origins are unclear and how they reached such enormous masses has been a challenge for astronomers to understand.
However, astronomers Fan Zou and W. Niel Brandt, both of Penn State University, led a team that connected the two black hole growth mechanisms using observations and simulations. The results could finally provide some answers.
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“A really big question is how do these supermassive black holes become so massive?” said Zou while presenting their work at the 244th meeting of the American Astronomical Society in Wisconsin. “To solve this problem, we need to track the overall growth history of these supermassive black holes.”
As mentioned, black holes grow via two main mechanisms. The first is due to the accretion of cold gas from their host galaxy. This gas forms an accretion disk around the black hole itself, and the material in the disk gradually spirals toward the black hole’s core. The accretion disk can become so dense that friction between gas molecules causes it to heat up to millions of degrees, thereby radiating X-rays. The other mechanism occurs during galaxy collisions. When this happens, not only do galaxies merge, but their supermassive black holes also eventually merge and release a burst of light. gravitational waves.
Hunting cosmic voids
To assess the extent to which gas accretion contributes to the growth of supermassive black holes, the study team combed through more than 20 years of archival data from NASAIt is Chandra X-ray Observatorythe European Space Agency program XMM-Newton mission and the ROSITE X-ray instrument on board joint German-Russian ship Spektr-RG spatialship. Researchers were able to identify X-ray signals coming from around 8,000 fast-growing supermassive black holes.
“When supermassive black holes accrete surrounding gas, they emit powerful X-rays. So by detecting them in the X-ray bands, we can measure their accretion power,” Zou said.
They then turned to IllustrisTNG cosmological supercomputer simulation to model galaxy mergers throughout cosmic history. From there, the team combined X-ray data showing growth by accretion with results from simulated mergers to better understand how and when supermassive black holes grew over the past 12 billion years, versus 1, 8 billion years after the creation of supermassive black holes. big Bang Until today.
These simulations “capture the overall large-scale structure (of the universe) but are also capable of probing individual galaxies,” Zou said.
Stories of supermassive black holes
Zou and Brandt found that radiological data show that accretion has been the primary driver of black hole growth in all epochs of cosmic history. Additionally, the more massive the galaxy, the faster the supermassive black hole inside it grows through accretion. Mergers, on the other hand, are less important drivers of black hole growth according to the simulations, but can still have some influence.
“In most cases, accretion dominates the growth of supermassive black holes and mergers make notable secondary contributions,” Zou said.
These results also show that supermassive black holes grew faster earlier in the universe, and that new ones appear frequently. However, by about 7 billion years ago, the total number of supermassive black holes had more or less stabilized and few new supermassive black holes were forming. Mergers had a greater impact in later history, reaching their peak importance in the growth of black holes around 4 billion years ago.
“We discovered that once the universe reached about 40% of its age, the overall demographics of supermassive black holes are very similar to the demographics of supermassive black holes that we observe in the local universe,” Zou said.
Astronomers have even specifically modeled our galaxy’s black hole, Sagittarius A*and concluded that it evolved most of its matter relatively late in the cosmic universe. time. This growth would have been mainly due to accretion, with the majority of mergers of the Milky Way with other galaxies having taken place more than 8 to 10 billion years ago. However, the European Space AgencyIt is Gaia Mission has recently found evidence for a dwarf galaxy that collided with the Milky Way only 2 to 3 billion years ago. Dwarf galaxies are thought to contain intermediate-mass black holes, measuring tens to hundreds of thousands of times the mass of our sun, and it is possible that one of them merged with Sagittarius A* to increase the mass of our black hole.
Since the results only take us back 1.8 billion years after the Big Bang, they don’t describe how the seeds of supermassive black holes first formed. This remains a dilemma for cosmologists, especially since The Hubble Space Telescope and the James Webb Space Telescope to have found surprisingly massive black holes very early in the history of the universe. How they grew up we do not currently know that it will represent millions of times the mass of our sun in less than a billion years.
An article describing the results was published in March in The Astrophysics Journalwith a second article pending.