Colliding Galaxies Create Supermassive Black Holes
October 23, 2013

Could Supermassive Black Holes Arise From Galaxy Collisions?

John P. Millis, PhD for - Your Universe Online

Normal black holes – a bit of a misnomer in and of itself as there is little that is “normal” about black holes of any type – are typically on the order of a few times the mass of our Sun or less. These objects are believed to result from the collapse of a massive star, an incredibly energetic event we call a Type II Supernova.

But there exists at the center of virtually every galaxy we have studied a black hole of such magnitude that the traditional black hole progenitor does not seem likely. These aptly named supermassive black holes can reach millions or billions of times the mass of our Sun.

It is incredibly unlikely such monsters are created by the collapse of supermassive stars, as the star in question would itself need to be millions or billions of solar masses. The most massive main sequence stars ever observed top out at a couple of hundred Suns. Thus it seems such massive black holes must be created by some other mechanism.

There are several possibilities: one proposal suggests if a sufficiently large gas cloud began to collapse, a black hole could readily be created, bypassing the star phase all together. Perhaps such a cloud would be present early on in the formation of a galaxy, which makes sense because we already know that the central black hole of a galaxy has a strong influence on the galaxy as a whole.

Yet, we’ve never observed such large clouds, and no one has yet been able to work out a theory explaining the dynamics of how the cloud could collapse into a supermassive black hole without also swallowing the rest of the young, still forming galaxy.

Yet another scenario suggests supermassive black holes don’t form in the usual way at all – that perhaps most of these objects are from a long forgotten era where dark matter was more dominant and pooled together as early stars formed. The annihilation of dark matter in the cores of such stars would produce such immense amounts of energy that very large stars could form around the dark matter – perhaps creating objects on the order of billions of solar masses. Ultimately, these stars would fail once the dark matter was self-consumed in the core and would ultimately implode to create a supermassive black hole.

These objects would no longer exist today, explaining why we don’t see such massive stars. But it also makes the hypothesis difficult to test.

Perhaps the most widely held position is that supermassive black holes form from the merger of smaller black holes. But new research suggests while this is possible, it may not be quite that simple.

The primary event that would bring large black holes together would be the collision and merger of entire galaxies. In such cases, the black holes would migrate toward the center of the new, larger galaxy. Eventually, the black holes would spiral around each other and collide, creating a new, more massive black hole.

While we see merging galaxies in our Universe, the process is so slow that we have yet to catch this process in its entirety. Luckily, however, we should be able to detect remnants of these mergers by studying gravitational waves.

In our Universe, energy is stored in fields, such as electric, magnetic and gravitational fields. When these energy fields are perturbed, they can emit waveforms that propagate across space. In the case of electric and magnetic fields, the waves take the form of photons of light. When massive objects oscillate, however, it is gravitational waves that are created.

Unfortunately, because gravity is so weak compared to electric and magnetic fields, the waves are much fainter and more difficult to detect than light. In fact, gravitational waves have only ever been marginally detected from outer space. But merging black holes should produce some of the largest fluxes of gravitational waves anywhere in the Universe.

Of course, there is still a complication. These mergers are taking place so far away, they would still be nearly impossible to directly detect on Earth. Thankfully, researchers have come up with a clever way to study the motion of gravitational waves moving across the cosmos: measuring pulsars.

While not quite as exotic as black holes, pulsars are rapidly rotating stellar remnants that are incredibly dense. Like black holes, these objects are primarily produced from the collapse of a massive star and are so compact that a 14-ounce soup can filled with pulsar material would have roughly the same mass as our Moon.

These objects rotate up to 1,000 times per second, allowing us to see regular pulsations from Earth as the beams of radiation from these rapid rotators sweep past our line of sight, similar to a light-house signaling ships at sea. This regular pulsing is like a cosmic metronome, allowing astronomers to learn all sorts of information about their local neighborhoods.

Teams of scientists are currently cataloging the pulsations from these objects all over the Universe and have created a database stretching back more than 10 years. The ultimate goal is to be able to image the gravitational wave field across the cosmos, but that requires more data. In the interim, a team led by Vikram Ravi at the University of Melbourne and Ryan Shannon at CSRIO, are using the data that is already collected to look for changes in pulsar timing that would indicate the presence of gravitational waves from black hole mergers.

The conclusion? Black hole mergers are not the answer. While this may be one method for producing supermassive black holes, it is not the dominant mechanism, as the pulsar timing does not show a significant enough fluctuation for this to be the cause.

Perhaps related mechanisms, such as the slow accretion of gas by black holes, is the primary driver, or even one of the previously mentioned theories. But, from these preliminary results, it seems the answer may not be as obvious as once believed.