Tuesday, 5 April 2016

How do you form a massive black hole binary? (Part 3)

This post brings to an end a three-part series of posts about how massive black hole binary systems form, such as those recently observed to merge by the LIGO gravitational wave detector. The first two posts discussed how such massive black holes could form and how a binary system of black holes could form in isolation. Here I'm going to talk about a much more efficient way to form binary black holes, and that's in a star cluster!

As we discussed last week, forming a binary black hole in isolation is very hard. There are lots of processes that could disrupt the binary system before both stars form black holes, and once the binary has fallen apart it's hard to put it back together. But not so in star clusters! In star clusters there are lots of (gravitational) interactions between stars (or black holes) that, under the right circumstances, can lead to the formation of new binary systems. In fact there have been many simulations to explore exactly this sort of process!

The dense and massive star cluster R136 in the 30 Doradus nebula. The bright blue dots are predominantly massive
OB-type stars, many of which will end their lives as black holes (Credit: HST).

The basic idea is a simple 3-step process first put forward by Sigurdsson & Hernquist in 1993. The first step is that star clusters are full of massive stars, and these massive stars are heavier than normal stars and so they naturally sink to the centres of the clusters (we observe this process, called mass segregation, in many star clusters). The second step is that once these stars reach the ends of their lives they explode as supernovae and leave behind stellar remnants such as black holes. This we also know to regularly take place.

Simulation showing the development of mass segregation in a star cluster after only 2 Myrs (right)
compared to a lack of mass segregation at the beginning of the simulation (left). The massive stars (big circles)
are clearly more centrally concentrated in the right-hand panel (Credit: Marc Freitag).

The third step, which is where the clustered environment of the star cluster is critical, is that close encounters between stars (or black holes) in the centres of star clusters can quickly lead to the formation of binary systems, and therefore potentially black hole binary systems. Outside of star clusters the density of stars in space is very low and close encounters are very rare (which is fortunate for our Sun and solar system otherwise we'd always be bumping into other stars!). But star clusters are dense and the stars within them are moving very fast, meaning that close encounters between stars are common. If the stars come close enough to each other in just the right way then a binary system forms.

Many studies have confirmed this general picture and have estimated that it should take (on average) about 1 Giga-year (1000 million years) for enough interactions to occur between stars and black holes to form a binary black hole system in a dense cluster. This means that such binary black hole systems are most likely to be found in older clusters, such as globular clusters, though it shouldn't be impossible to find them in younger clusters.

Does the Omega Centauri globular cluster house
a central black hole? (Credit: ESO)
There is good evidence that this happens quite often. A recent study of the massive globular cluster Omega Centauri found that the dynamics of its stars suggest a particularly massive central black hole, while another study even found evidence that there might be a black hole within the nearby Orion Nebula Cluster. Confirming these theories is difficult because directly detecting black holes isn't always easy, but the evidence is there to suggest they might be occupying many of our favourite star clusters!

Further dynamical interactions within the star cluster can lead to two important effects for the binary black hole system. The first is that the binary will shrink over time due to dynamical interactions with individual stars - this brings the black holes in the binary closer together, making an eventual merger more likely. The second effect is that these black hole binary systems can be easily ejected from the star cluster, often at quite high velocities, through interactions with other binary systems.

The most likely clusters to produce binary black hole mergers are intermediate age clusters with ages of a few Giga-years. Younger clusters are unlikely to have had enough time to form the binary black hole systems, while older clusters will have formed them but they might have all merged by now.

This process implies that a typical star cluster, which contains many massive stars, will easily produce many black holes, and most likely a few black hole binary systems that will one day merge and release gravitation waves just like those observed in September.

Given how many star clusters there are in the Universe its hardly surprisingly that such a black hole merger was observed so easily!

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