Astronomers Hunt The Wandering Black Holes

Wandering supermassive black holes – those that don’t lie at the center of your galaxies – can be hard to find, but not all roving black holes are lost! A new study demonstrates how we can hope to discover these missing nomads in the future.

We know that the center of each massive galaxy hosts a supermassive black hole weighing millions to billions of solar masses. But galactic centers aren’t the only place supermassive black holes can hide! In fact, we expect most galaxies to host many more of these monsters than the central supermassive black holes. Why? Because galaxies merge.

The structure in our universe is largely built hierarchically: over time, galaxies often collide with one another, growing progressively larger with each merger. But with each of these mergers, at least two supermassive black holes – one from each of the merging galaxies – are introduced into the resulting turbulence.

While gas and stars reorder perfectly in a new galaxy, eventually erasing all evidence of fusion, black holes don’t behave so well. In fact, simulations show that it could take billions of years for these supermassive black holes to make their way to the center of the newly formed galaxy and merge – if at all!

As more galaxy collisions occur, more off-center “wandering” supermassive black holes are produced – and currently galaxies can host dozens of black holes over a million solar masses. So how do we find this vast population of wanderers? A new study led by Angelo Ricarte (Center for Astrophysics | Harvard & Smithsonian; Black Hole Initiative) explores the possibilities.

Ricarte and collaborators use a set of cosmological simulations called ROMULUS to produce a realistic expectation of the black holes lurking in our universe. These simulations carefully track the positions and dynamics of supermassive black holes as galaxies merge and evolve over time, allowing us to explore the population of roving supermassive black holes predicted to arise in galaxies at different times in the universe.

From this simulated population, the authors predict the ways in which these wanderers can betray their locations:

Hyperluminous X-ray sources – Some nearby errant black holes that accumulate should be detected as excessively bright X-ray sources.

Dual active galactic nuclei – Simulations predict that galaxies often host more than one supermassive black hole that builds up dramatically – particularly at higher redshifts.

X-ray halo – If black holes are too far away or too dark to resolve individually, we can identify strays by stacking images of galaxies of similar mass. The “halo” of excess X-ray radiation can then be used to describe the population of errant black holes.

Tide Break Events – Wandering supermassive black holes can destroy stars that come too close! These interruptions must produce transient signals displaced from the galactic centers.

ROMULUS simulations show that for black holes smaller than 10 billion solar masses, nomads far outnumber the central supermassive black holes in our universe. The work of Ricarte and colleagues demonstrates that we will need to consider this nomadic population carefully as we analyze our observations of the universe.

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The Hunt for Wandering Black Holes