Supermassive black holes tend to sit, more or less stationary, in the centers of galaxies. But not all of these incredible cosmic objects stay put; some may fall irregularly, oscillating around galaxies like cosmic nomads.
We call these black holes “vagabonds,” and they are largely theoretical, as they are difficult (but not impossible) to observe and therefore quantify. But a new set of simulations has allowed a team of scientists to figure out how many travelers there should be and where they would be, 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 the mass of our Sun) form and grow, a process shrouded in mystery.
Cosmologists believe that supermassive black holes (SMBH) reside in the nuclei of all or at least most galaxies in the universe. The masses of these objects are usually approximately proportional to the mass of the central galactic bulge surrounding them, suggesting that the evolution of the black hole and its galaxy are related in some way.
But the pathways of supermassive black hole formation are unclear. We know that black holes of stellar mass are formed from the collapse of the massive star core, but this mechanism does not work for black holes more than 55 times the mass of the Sun.
Astronomers think that SMBHs grow by accreting stars, gas, and dust, and fuse with other black holes (very thick in the nuclei of other galaxies, when these galaxies collide).
But cosmological time scales are very different from our human time scales, and the process of colliding two galaxies can take a long time. This means that the potential window for fusion can be disrupted quite large and the process can be delayed or even avoided altogether, resulting in these “travelers” of the black hole.
A team of astronomers led by Angelo Ricarte of the Harvard & Smithsonian Center for Astrophysics has used Romulus’ cosmological simulations to estimate how often it should happen in the past and how many black holes would still remain today.
These simulations track the orbital evolution of pairs of supermassive black holes, meaning they are able to predict which black holes will reach the center of their new galactic home and how long that process should take, too. like how many never get there.
“Romulus predicts that many supermassive black hole binaries will form after several billion years of orbital evolution, while some SMBHs will never reach the center,” the researchers wrote in their article.
“As a result, massive galaxies from the Milky Way to Romulus are found to host an average of 12 supermassive black holes, which usually roam the halo away from the galactic center.”
At the beginning of the Universe, nearly 2 billion years after the Big Bang, the team found that vagrants overcome and surpass the supermassive black holes in galactic nuclei. This means that they would produce most of the light we would expect to shine from the material surrounding the active SMBHs, glowing intensely as it orbits and accreting into the black hole.
They are kept close to their seed mass (that is, the mass in which they formed) and probably originate in smaller satellite galaxies orbiting larger ones.
And some travelers should still be there today, according to the simulations. In fact, there should be many in the local universe.
“We found that the number of stray black holes scales approximately linearly with the halo mass, so we expect thousands of stray black holes in the halos of galaxy clusters,” the researchers wrote.
“Locally, these travelers account for about 10 percent of the local black hole mass budget once the seed masses are accounted for.”
These black holes may not necessarily be active and therefore would be very difficult to detect. In a future paper, the team will explore in detail the possible ways to observe these lost wanderers.
So all we have to do is find the black holes of stellar mass and intermediate mass lost …
The research has been published in Monthly notices from the Royal Astronomical Society.