The story of the dark hearts of galaxies has been told almost entirely for the first time, as astronomers combined X-ray observations with detailed supercomputer models to chart the growth of supermassive black holes over 12 billion years of cosmic history.
In this way, scientists have shown that the black hole at the core of our Milky way galaxy reached its four million solar masses relatively late in its history.
Supermassive black holes range from millions of times more massive than our sun up to billions of times more massive, but their origins are unclear and how they grew to such enormous masses is a challenge for astronomers to understand.
Now, however, astronomers Fan Zou and W. Niel Brandt, both of Penn State University, led a team that linked the two black hole growth mechanisms from observations and simulations. The results may finally provide some answers.
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“A very big question is how do these supermassive black holes get so massive?” Zu said while presenting their work at the 244th meeting of the American Astronomical Society in Wisconsin.. “To address this, we need to trace the entire growth history of these supermassive black holes.”
As mentioned, black holes grow through two main mechanisms. One is by accreting 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 become so dense that the friction between gas molecules causes it to heat up to millions of degrees, emitting X-rays in the process. The other mechanism occurs during galaxy collisions. When this happens, not only do the galaxies merge, but their supermassive black holes also eventually merge and release a burst of gravitational waves.
Tracking space gaps
To assess how much gas accretion contributes to the growth of supermassive black holes, the research team searched more than 20 years of archival data from NASA‘c Chandra X-ray Observatoryof the European Space Agency XMM-Newton the mission and EROSITA X-ray machine on board the joint German-Russian ship Spectrum-RG spacecraft. The researchers were able to identify X-ray signals coming from about 8,000 rapidly accreting supermassive black holes.
“When supermassive black holes accrete the surrounding gas, they emit strong X-rays, so by detecting them in the X-ray bands, we can measure their accretion strength,” Zou said.
Then they turned to IllustrisTNG cosmological supercomputer simulation to model galaxy mergers throughout cosmic history. From there, the team combined X-ray data showing accretion growth 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 after Big bang until today.
These simulations “capture the overall large-scale structure [of the universe] but they can also probe individual galaxies,” Zou said.
Stories of supermassive black holes
Zou and Brandt found that the X-ray data show that accretion has been the dominant driver of black hole growth throughout all epochs of cosmic history. Also, the more massive the galaxy, the faster the supermassive black hole inside grows by accretion. Mergers, on the other hand, are less prominent drivers of Black hole growth according to the simulations, but may still have some influence.
“Accretion dominates the growth of the supermassive black hole in most cases, and mergers make some notable secondary contributions,” Zou said.
These results also show that supermassive black holes grew faster earlier in the universe, with new ones appearing frequently. About 7 billion years ago, however, the total number of supermassive black holes more or less stabilized with the formation of a few new supermassive black holes. Mergers were more influential in later history, peaking in importance for the growth of black holes about 4 billion years ago.
“We discovered this once The 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,” Zou said.
Astronomers even specifically modeled our galaxy’s black hole, Sagittarius A*and concluded that most of its matter grew relatively late in space time. This growth would be mainly through accretion, with most of the Milky Way’s mergers with other galaxies occurring over 8 billion to 10 billion years ago. whatever European Space Agency‘c Mission Gaia there is 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 merged with Sagittarius A* to increase the mass of our black hole.
Because the results only take us back to 1.8 billion years after the Big Bang, they do not describe how the seeds for supermassive black holes first formed. This remains a quandary for cosmologists, especially as Hubble Space Telescope and on James Webb Space Telescope i found surprisingly massive black holes much earlier in the history of the universe. How they grew up to be millions of times the mass of our sun in less than a billion years is currently unknown.
A paper describing the findings was published in March in The Astrophysical Journalwith a second paper pending in preparation.
