The black hearts of galaxies have been told in almost full detail for the first time, as astronomers combined X-ray observations with detailed supercomputer models to chart the growth supermassive black holes more than 12 billion years of the universe’s history.
In doing so, scientists proved that the black hole at the core of our Milky way galaxy it 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 a billion times more massive, but their origin is unclear and it has been difficult for astronomers to understand how they grew to such huge masses.
But now astronomers Fan Zou and W. Niel Brandt, both of Penn State University, led a team that linked the two mechanisms of black hole growth from observations and simulations. The results may finally provide some answers.
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“The very big question is how do these supermassive black holes grow so massive?” Zou said while present your work at the 244th meeting of the American Astronomical Society in Wisconsin.. “To address this, we need to follow the overall growth history of these supermassive black holes.”
As previously mentioned, black holes grow through two main mechanisms. One is the accretion of cold gas from their host galaxy. This gas forms an accretion disk around the black hole itself, and matter from the disk gradually spirals toward the black hole’s core. An accretion disk can grow so dense that the friction between gas molecules causes it to heat up to millions of degrees, emitting X-rays in the process. The second mechanism occurs in galaxy collisions. When this happens, the galaxies will not only merge, but their supermassive black holes will also eventually merge and release an explosion gravitational waves.
Tracking the voids of space
To assess how much gas accretion contributes to the growth of supermassive black holes, the study team examined over 20 years of archival data. NASA‘with the Chandra X-ray ObservatoryEuropean Space Agency XMM-Newton mission a eROSITY X-ray machine on board the joint German-Russian Spektr-RG space ship. Scientists were able to identify X-ray signals coming from about 8,000 rapidly accreting supermassive black holes.
“When supermassive black holes accumulate surrounding gas, they emit strong X-rays, so by detecting them in the X-ray bands, we can measure their accretion power,” Zou said.
Then they turned to IllustrisTNG cosmological a supercomputer simulation for modeling galaxy mergers throughout the history of the universe. From there, the team combined X-ray data showing growth through accretion with the results of simulated mergers to understand how and when supermassive black holes grew over the past 12 billion years, from 1.8 billion years to Big Bang until today.
These simulations “capture the overall large-scale structure [of the universe] but they are also able to probe individual galaxies,” Zou said.
Stories about supermassive black holes
Zou and Brandt found that X-ray data show that accretion has been the dominant driver of black hole growth during all epochs of cosmic history. Moreover, the more massive the galaxy, the faster the supermassive black hole grew inside the accretion. Mergers, on the other hand, are less significant drivers Black hole growth according to the simulations, but may still have some effect.
“In most cases, accretion dominates the growth of supermassive black holes, and mergers make some remarkable secondary contributions,” Zou said.
These results also show that supermassive black holes previously grew faster in the universe and new ones appeared frequently. However, around 7 billion years ago, the total number of supermassive black holes more or less stabilized and several new supermassive black holes formed. Mergers had a greater impact later in history, peaking in importance for black hole growth about 4 billion years ago.
“We found that once universe reaches about 40% of its age, the overall demography of supermassive black holes is very similar to the demography of supermassive black holes we see in the local universe,” said Zou.
Astronomers have even specially modeled our galaxy’s black hole, A* scorerand concluded that most matter accreted relatively late in the universe time. This growth would be primarily due to accretion, with most mergers of the Milky Way with other galaxies occurring over 8 billion to 10 billion years ago. However, European Space Agency‘with The Gaia Mission has recently found evidence for a dwarf galaxy that collided with the Milky Way just 2 billion 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 may have 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 do not describe how the seeds of supermassive black holes first formed. This remains a problem for cosmologists, especially as Hubble Space Telescope and The James Webb Space Telescope found surprisingly massive black holes very early in the history of the universe. How they grew to be a million times the mass of our Sun in less than a billion years is not currently known.
One paper describing the findings was published in March The Astrophysical Journalwith a second paper waiting in the pipeline.