Looking into the Past with Gravitational Waves and Spinning Black Holes

We are truly in an amazing era of observational astronomy with discovery and progress in gravitational-wave astronomy.  Recently we had the announcement of a new set of 35 gravitational wave events for the latest observation period (O3b), the catalogue of events called GWTC-3. To think of this another way, during this observation period we were detecting these cataclysmic collisions between black holes and/or neutron stars in space, billions of light-years away, at least every week (on average).   

The majority of the gravitational wave events detected so far are from pairs of black holes merging in space-time, and with every detection, we gain information about the masses of the objects, how fast they were spinning and even which direction they were spinning (relative to their orbital plane). With this growing catalogue of binary black holes, we can study the population properties of these compact objects, giving us insights into how they formed and evolved.

Black holes are a type of compact remnant object, which are the densest objects in the Universe. While we refer to them as objects, they are really regions in space where the gravitational pull is so strong that not even light can escape.  

While they do not emit light, astronomers have made observations of black holes through electromagnetic observations by looking at objects they interact with that do emit light - such as stars and clouds of gas that orbit them, or other objects that form part of a binary system with them. We can also detect pairs of black holes using gravitational waves.

But how do we form pairs of these compact objects?

There are two main pathways to form a binary black hole: the first scenario involves two stars already in a binary, living their best lives until each one dies in a core-collapse supernova to form black holes. These stars need to be relatively massive (usually, well above 20 solar masses during their progenitor phases). The evolution pathway is known as the isolated or field formation scenario. 

The second is where you have regions in space where there are many black holes in close proximity, such as what would be expected inside globular clusters, where black holes are readily interacting, leading to binary pairs of black holes capturing each other through their mutual gravitation. This is referred to as dynamical evolution.

These pathways have distinct features in what we expect the spin distribution of these binary black holes to look like. Black holes in binaries formed via isolated evolution tend be spinning in the same direction as the orbital motion of the binary whereas dynamically formed systems have spins that are randomly orientated. These are distributions we can measure using gravitational-wave observations.
 

Recent studies have explored the spin properties of the population of binary black holes, drawing different conclusions about the population. One study found evidence that supported both isolated and dynamical formation, whereas the other suggests that the population could be fully consistent with just the isolated formation scenario.

So which analysis is correct?

Well, the answer really depends on the models we use to analyse our population. When gravitational-wave astronomers analyse a population of black holes, they need a model to fit the population. These models need to be ‘good’ models. Usually, we can check if a model is ‘bad’ by comparing the fit we get to the observations we have made. However, it turns out our handy checks are tricked when there are features in our population that are considered sharp. An example of a sharp feature is something like a narrow peak in our spin distribution of binary black holes.

To resolve this conflict in recent literature, my co-authors and I adapted the models used by the LIGO-Virgo collaboration to be more flexible. The old models could not account for a sub-population of binaries with spin very close to zero (i.e. not spinning) even though some previous studies suggest that there could be an excess of black holes with negligible spin. 

We analysed a population of 44 binary black hole mergers, finding evidence for two sub-populations within the spin distribution of black hole binaries. The majority of the populations (80%) have negligible spins while the other has moderate spin with a preference for smaller tilt angles. This result can be fully explained with the isolated formation scenario.

So what does this mean astrophysically? 

It outlines that the majority of progenitor stars that formed these black holes would lose their angular momentum when the star’s envelope is removed (during a prior evolutionary phase) by its companion in the binary, forming black hole pairs with very little spin. There may also be a small fraction (around 20%) of binaries where the second-born black hole is spun up through tidal effects.

This has been a cautionary tale for gravitational-wave population analysts such as myself. We need to make sure our models are flexible enough to allow us to capture even the sharpest of features, otherwise, we might draw incorrect conclusions. 

Our work opens a number of interesting avenues to explore, such as seeing if there is a relationship between the mass and spin of these different sub-populations. Investigating such correlations can help us improve our models and allow us to better distinguish between different formation scenarios of binary black holes.