The centers of galaxies are where supermassive black hole tend to be more or less stationary. These amazing cosmic objects may not stay put. Some may wander around galaxies as cosmic nomads.
These black holes are called 'wanderers'. They're difficult to quantify and observe. A new set of simulations allowed scientists to calculate how many wanderers should exist and their locations. This could be used to help identify them in the Universe.
This could have significant implications for understanding how supermassive dark holes, monsters millions to trillions of times larger than our Sun, form and grow. A process that remains a mystery.
Cosmologists believe that supermassive dark holes (SMBHs), reside in the nuclei all or most galaxies in our Universe. The masses of these objects are often roughly proportional to their central galactic bulge mass, suggesting that evolution of the black hole is somehow connected to its galaxy.
The formation paths of supermassive dark holes are not clear. While we know that stellar-mass black hole formation occurs from the core collapse massive stars, this mechanism is not applicable to black holes with a mass greater than 55 times that of the Sun.
Astronomers believe that SMBHs are formed by the accretion stars, gas, and dust and then mergers with black holes (very chunky nuclei at other galaxies when these galaxies collide).
However, cosmological timescales can be very different to our human timescales and it is possible for two galaxies to collide very slowly. The potential for the merger to be interrupted is large. This could lead to delays or even prevent the entire process from happening.
Angelo Ricarte, an astronomer at the Harvard & Smithsonian Center for Astrophysics, used the Romulus cosmological simulators to calculate how often this should have happened in the past and how many black holes might still be roaming today.
These simulations track the orbital evolutions of supermassive black hole pairs, so they can predict when and how long it will take to reach the center.
The researchers wrote that Rotus predicted that supermassive black holes binaries would form after billions of years orbital evolution, and some SMBHs would never make it to center."
"As such, Milky Way-mass Galaxies in Romulus were found to have an average of 12 supermassive dark holes. These black holes wander the halo far away from the galactic center."
The team discovered that wanderers outnumbered and outshone supermassive black hole in galactic nuclei in the early Universe. This was before the Big Bang. They would be able to produce the majority of the light that we expect to see from active SMBHs. This is because the material orbits the black hole and accretes onto it, shining brightly.
They are close to their seed mass, which is the mass at which it formed. They probably originated in smaller satellite galaxies orbiting larger ones.
According to simulations, there should be some wanderers around today. There should be quite a few people living in the local Universe.
Researchers wrote that they found that wandering black hole numbers are linearly related to halo mass. This means that there will be thousands of wandering blackholes in galaxy cluster halos.
"Locally these wanderers account approximately 10 percent of local black hole mass budget after seed masses have been accounted for."
These black holes might not be necessarily active and would therefore be difficult to spot. The team will explore in detail all possible ways to observe these wanderers in a forthcoming paper.
All that is left now is to find the intermediate-mass black hole and lost stellar-mass black holes.
The research was published in the Monthly Notices of the Royal Astronomical Society.
