Supermassive black holes are usually located more or less stationary in the centers of galaxies. But not all of these amazing cosmic objects stay in place; some of them fly around galaxies like space travelers.
We call such black holes “wanderers,” and they are largely theoretical because they are difficult (but not impossible) to observe and therefore quantify. But a new set of simulations has allowed a group of scientists to figure out how many wanderers there must be and where they are–which, in turn, may help us detect them in the universe.
This could have important implications for our understanding of how supermassive black holes — monsters with a mass millions and billions of times that of our Sun — form and grow, a process that is shrouded in mystery.
Cosmologists believe that supermassive black holes (SMBHs) are found in the nuclei of all – or at least most – galaxies in the Universe. The mass of these objects is usually roughly proportional to the mass of the central galactic bulge around them, suggesting that the evolution of a black hole and its galaxies are somehow related.
However, the ways of formation of supermassive black holes are unclear. We know that stellar-mass black holes are formed by the collapse of the cores of massive stars, but this mechanism does not work for black holes whose mass exceeds the mass of the Sun by about 55 times.
Astronomers believe that SMBHs grow through accretion of stars, gas and dust, and mergers with other black holes (very large ones in the cores of other galaxies when these galaxies collide).
But cosmological timelines are very different from our human timelines, and the process of two galaxies colliding can take a very long time. This makes the potential window for disruption of the merger process quite large, and the process could be delayed or even prevented entirely, leading to these “wanderers”-black holes.
A team of astronomers led by Angelo Ricarte of the Harvard and Smithsonian Centers for Astrophysics used Romulus cosmological simulations to estimate how often this has happened in the past, and how many black holes would wander today.
These simulations independently track the orbital evolution of pairs of supermassive black holes, which means they can predict which black holes are likely to reach the center of their new galactic home and how long the process will take, as well as how many will never get there.
“Romulus predicts that many supermassive black hole binaries will form after several billion years of orbital evolution, and some SMBHs will never reach the center,” the researchers wrote in their paper.
“As a result, it appears that galaxies with the mass of the Milky Way have an average of 12 supermassive black holes in Romulus, which typically roam in the halo region far from the galactic center.”
In the early Universe, up to about 2 billion years after the Big Bang, the team found, wandering supermassive black holes in galactic cores both outnumber and outshine them. This means that they produce most of the light we expect to see from the material around active supermassive black holes, glowing brightly as it orbits and flushes against the black hole.
They remain close to their initial mass — that is, the mass at which they formed — and probably come from small satellite galaxies orbiting larger galaxies.
And some wanderers, according to the simulations, must still exist today. There should be quite a few of them in the local universe.
“We found that the number of wandering black holes is roughly linearly related to the mass of the halo, so we expect thousands of wandering black holes in the halo of galaxy clusters,” the researchers write.
“At the local level, these wandering holes account for about 10 percent of the local black hole mass distribution after accounting for stellar mass.”
These black holes may not necessarily be active, making them very difficult to detect. In upcoming work, the team will look in detail at possible ways to observe these lost wanderers.
Then we will only have to find lost stellar-mass and intermediate-mass black holes…
The study was published in Monthly Notices of the Royal Astronomical Society.
would some one please update their ideas (with actual science). Old hog wash is still hog wash and it smells like hog