Supermassive black holes are usually located more or less stationary in the centers of galaxies. But not all of these amazing space objects remain in place; some of them fly through 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 allowed a team of scientists to figure out how many wanderers there should be, and where they are - which, in turn, could help us find them in the universe.
This could have important implications for our understanding of how supermassive black holes form and grow - monsters millions and billions of times the mass of our Sun - a process shrouded in mystery.
Cosmologists believe that supermassive black holes (SMBHs) are found in the cores 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 the black hole and its galaxy is somehow related.
However, the pathways for the formation of supermassive black holes are unclear. We know that stellar mass black holes are formed by the collapse of the core of massive stars, but this mechanism does not work for black holes whose mass is about 55 times the mass of the Sun.
Astronomers believe that SMBHs grow from accretion of stars, gas and dust, and mergers with other black holes (very large in the cores of other galaxies when these galaxies collide).
But cosmological timelines are very different from our human timelines, and it can take a very long time for two galaxies to collide. This makes the potential window for disrupting the fusion process quite large, and the process can be delayed or even completely prevented, leading to the appearance of these "wanderers" - black holes.
A group of astronomers led by Angelo Ricarte of the Harvard and Smithsonian Astrophysics Centers used Romulus cosmological simulations to estimate how often this 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 are able to predict which black holes are most likely to make it to the center of their new galactic home, how long it will take, and how many of them will never get there. …
"Romulus predicts that many supermassive black hole binars will form after billions of years of orbital evolution, and some SMBHs will never make it to the center," the researchers write in their paper.
"As a result, it turned out that in galaxies with the mass of the Milky Way, Romulus has an average of 12 supermassive black holes, which usually wander 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 discovered that wandering supermassive black holes in galactic cores both outnumber and eclipse them. This means that they produce most of the light we expect to see from material around active supermassive black holes, glowing brightly as it orbits and flows towards the black hole.
They remain close to their initial mass - that is, the mass at which they formed - and likely originate from small satellite galaxies that orbit larger galaxies.
And some wanderers, according to modeling, should exist today. There should be quite a lot of them in the local Universe.
"We found that the number of wandering black holes is roughly linear with the mass of the halo, so we expect thousands of wandering black holes in the halos of galactic clusters," the researchers write.
"At the local level, these wandering holes account for about 10 percent of the local mass distribution of black holes after accounting for the mass of stars."
These black holes may not necessarily be active and therefore very difficult to detect. In the upcoming work, the team will look in detail at possible ways of observing these lost wanderers.
Then we will only have to find the lost black holes with stellar mass and intermediate mass …
The study was published in the Monthly Notices of the Royal Astronomical Society.