Hugh black hole They tend to sit, quite still, at the center of the galaxy. But not all of these beautiful cosmic objects stay where they are; Some may be tilted, tottering around the galaxy like cosmic wanderers.
We call such black holes “explorers”, and they are mostly theoretical, because they are difficult (but not impossible) to observe, and therefore to measure. But a new series of simulations has allowed a team of scientists to determine how many wanderers and where they are – which in turn could help us identify them in the universe.
This could have important implications for our understanding of how supermassive black holes – monsters millions to billions of times in mass – form and grow, a process shrouded in mystery.
Cosmologists believe that supermassive black holes (SMBHs) are at the core of all – or at least most – of the galaxies in the universe. The masses of these objects are usually roughly proportional to the mass of the surrounding central galaxy bulge, which suggests that the evolution of black holes and their galaxies is somehow connected.
But the path of formation of supermassive black holes is unclear. We know that stellar-mass black holes form from the main collapse of massive stars, but this mechanism doesn’t work for nearby black holes. 55 times the mass of the sun.
Astronomers believe that SMBH grows by collecting stars, gas, and dust, and merging with other black holes (very thick holes in the cores of other galaxies, when the galaxies collide).
But the cosmic time scale is very different from the human time scale, and the collision of two galaxies can take a very long time. This makes the potential window for merging very large, and the process can be delayed or even prevented altogether, resulting in the black hole ‘wandering around’.
A team of astronomers led by Angelo Ricciarti of Harvard and the Smithsonian Center for Astrophysics used Romulus cosmic simulations to estimate how often this should have happened in the past, and how many black holes still roam today.
The simulation automatically tracks the orbital evolution of the supermassive black hole pair, meaning it can predict which black holes might reach the center of their new galactic habitat, and how long that process will take — as well as how many never get there.
“Romulus predicts that many supermassive black hole binaries form after several billion years of orbital evolution, while some SMBHs will never reach their center.” The researchers wrote in their paper.
As a result, the Milky Way galaxy in Romulus was found to have an average of 12 supermassive black holes, which usually wander in a halo away from the galactic center.
In the early universe, about 2 billion years ago big explosionThe team found that the number of wanderers outnumbered and outnumbered the supermassive black holes at the galactic core. That means it will produce most of the light we expect to see sparkle from the material around an active SMBH, shining brightly as it rotates and accumulates in the black hole.
It remains close to its seed mass – the mass that formed it – and may have originated from a smaller satellite galaxy orbiting a larger galaxy.
Some of the pedestrians should still be there today, according to simulations. In the local universe, there are bound to be some things hanging around.
“We found that the number of wandering black holes is roughly proportional to the mass of the corona, so we estimate thousands of wandering black holes in the galaxy cluster’s halos.” Researchers write.
“Locally, these wanderers represent about 10 percent of the local black hole mass budget after seed mass is taken into account.”
These black holes are not necessarily active, and therefore will be very difficult to detect. In a future paper, the team will explore in detail the ways in which we can monitor these lost travelers.
So all we have to do is find the missing stellar and middle-mass black holes…
Search published in Monthly Notices of the Royal Astronomical Society.
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