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Supermassive black holes tend to sit, more or less stationary, in the center of galaxies. But not all of these awesome cosmic objects stay in place; some can be turned awry, swinging around galaxies like cosmic nomads.
We call these black holes “wanderers”, and they are largely theoretical, because they are difficult (but not impossible) to observe, and therefore to quantify. But a new set of simulations have allowed a team of scientists to determine the number of wanderers and their locations, which in turn could help us identify them in the Universe.
This could have important implications for our understanding of how supermassive black holes – from monsters millions to billions of times the mass of our Sun – form and develop, a process that is shrouded in mystery.
Cosmologists believe that supermassive black holes (SMBHs) reside in the nuclei of all – or at least most – galaxies in the Universe. The masses of these objects are usually roughly proportional to the mass of the central galactic bulge that surrounds them, suggesting that the evolution of the black hole and its galaxy are somehow related.
But the routes of formation of supermassive black holes are unclear. We know that stellar-mass black holes form from the collapse of the nucleus of massive stars, but this mechanism does not work for black holes larger than 55 times the mass of the Sun.
Astronomers believe that SMBHs develop through accretion of stars, gas and dust, and mergers with other black holes (very large at the nuclei of other galaxies, when these galaxies collide) .
But cosmological timescales are very different from our human timescales, and the collision process of two galaxies can take a very long time. This makes the potential window of disruption large enough for the merger, and the process could be delayed or even completely prevented, resulting in these “stray” black holes.
A team of astronomers led by Angelo Ricarte of the Harvard & Smithsonian Center for Astrophysics used Romulus’ cosmological simulations to estimate how often this should have happened in the past and how many black holes still roamed today.
These simulations consistently track the orbital evolution of pairs of supermassive black holes, meaning that they are able to predict which black holes are likely to travel to the center of their new galactic home, and how long this process is expected to take. take – as well as how many never get there.
“Romulus predicts that many supermassive black hole binaries will form after billions of years of orbital evolution, while some SMBHs will never reach the center,” the researchers wrote in their paper.
“As a result, the Milky Way’s mass galaxies in Romulus host an average of 12 supermassive black holes, which generally travel the halo far from the galactic center.”
At the start of the Universe, about 2 billion years after the Big Bang, the team discovered that wanderers both outnumber supermassive black holes in galactic nuclei and outnumber supermassive black holes. This means that they would produce most of the light that we would expect to see shine from the material around active SMBHs, glowing brightly as it orbits and accumulates on the black hole.
They stay close to their original mass – that is, the mass at which they formed – and likely originate from smaller satellite galaxies that orbit larger ones.
And some wanderers should still be around today, according to the simulations. In the local universe, there should actually be quite a few lying around.
“We find that the number of stray black holes evolves roughly linearly with the mass of the halo, so we would expect thousands of stray black holes in the halos of galaxy clusters,” the researchers wrote.
“Locally, these wanderers account for about 10% of the local black hole mass budget once the seed masses are taken into account. “
These black holes are not necessarily active and would therefore be very difficult to spot. In a future article, the team will explore in detail possible ways to observe these lost wanderers.
Then all we have to do is find the lost black holes of stellar mass and intermediate mass …
The research was published in the Monthly notices from the Royal Astronomical Society.
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