A large number of errant supermassive black holes are roaming the Universe

 Supermassive black holes tend to be more or less stationary at the center of galaxies.  But not all of these incredible cosmic objects remain immobile;  some can get loose, navigating around galaxies like cosmic nomads.

 We call these black holes 'wandering', and they are largely theoretical because they are difficult (but not impossible) to observe and therefore to quantify.  But a new set of simulations allowed a team of scientists to figure out how many of these travelers there must be and their whereabouts – which, in turn, could help us identify them back 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 that is shrouded in mystery.

 Cosmologists think that supermassive black holes reside at the core of all – or at least most – galaxies in the Universe.  The masses of these objects are generally more or less proportional to the mass of the central galactic bulge around them, suggesting that the evolution of the black hole and its galaxy are somehow linked.

 But the pathways to supermassive black hole formation are unclear.  We know that stellar-mass black holes form from the collapse of the core of massive stars, but this mechanism does not work for black holes that are more than 55 times the mass of the Sun.

 Astronomers think that supermassive black holes develop through the accumulation of stars, gas and dust, and fuse with other black holes (very big ones in the cores of other galaxies when those galaxies collide).

 But cosmological timescales are very different from human timescales, and the process of colliding two galaxies can take a long time.  This makes the window for the fusion to potentially be stopped quite large, and the process could be delayed or even avoided altogether, resulting in these 'wandering' black holes.

 A team of astronomers led by Angelo Ricarte of the Harvard & Smithsonian Center for Astrophysics used the Romulus set of cosmological simulations to estimate how often this should have happened in the past, and how many black holes would still be roving today.

 As a result, the Milky Way galaxies in Romulus have been found to host an average of 12 supermassive black holes, which normally roam the halo far from the galactic center.”

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