Malcolm in the Middle

NASA’s Goddard Space Flight Center/Scott Noble

There are three known types of black holes: supermassive black holes that lurk in the hearts of galaxies, stellar mass black holes formed from stars that die as supernovae, and intermediate mass black holes with masses between the two extremes. It’s generally thought that the intermediate ones form from the mergers of stellar mass black holes. If that is true, there should be a forbidden range between stellar and intermediate masses. A range where the mass is too large to have formed from a star but too small to be the sum of mergers. But a new study of data from LIGO suggests that there are black holes in that forbidden range.¹

Only large stars can become supernovae. Based on the models we have, the smallest supernovae are produced by stars of around 10 solar masses. For these, the collapse of their cores likely produces neutron stars rather than black holes. Based on observations, the lightest black holes are around 3-4 solar masses, produced by a star of about 20 solar masses. Supernovae only occur for stars up to about 50 solar masses. Above that mass, a star collapses into a black hole almost directly. The gravity is so strong that the star can’t produce a bright explosion.

But then, around 150 solar masses stars, explode as hypernovae. They are also known as pair-instability supernovae. The collapse of their cores releases so much energy that the high-energy gamma rays they produce generate electron-positron pairs. Needless to say, hypernovae are much more violent than regular supernovae, which means much less of the central mass collapses into a black hole. The very first stars were likely tremendous beasts with masses greater than 300 Suns, and they would have produced black holes of more than 120 solar masses.

All of this means that stellar-mass supernovae would produce black holes in the 4-50 solar mass range, and the largest hypernovae stars would produce black holes above 120 solar masses, give or take. Stellar-mass black holes shouldn’t be possible in the 50-120 solar mass range. If we find black holes in that range, so our understanding goes, they must have been formed from the merging of smaller black holes.

So here’s where it gets interesting. It takes a lot of time for black holes to spiral into each other and merge, particularly for smaller black holes. While it is likely that mergers have produced black holes in this excluded range, it isn’t likely that they would experience a second merger within the lifetime of the cosmos. In other words, when gravitational telescopes such as LIGO detect black hole mergers, it is extremely unlikely that one of the original black holes would be in the excluded mass range. Finding such “lite” mergers would suggest there is something about the formation of black holes we don’t understand.

This is where the new study comes in. The authors looked at black hole mergers detected by the third run of the LIGO-Virgo gravitational wave observatories and ran statistics on the likely masses of the original black holes. One of the challenges with gravitational wave astronomy is that the mergers we observe are at the very limit of detection. To pull the signal out of the noise, astronomers have to run model simulations to see what best fits. The results are a bit fuzzy, which means our measurements of the black hole masses are a bit uncertain.

The team focused on 11 detection events and found that 5 of the mergers had a better than 75% chance that one of the original black holes was in the mass range of 50-120 solar masses. There are some issues with data noise that might skew the statistics, but the data still supports the existence of black holes in the excluded range. These may have formed from earlier black hole mergers, but not likely.

So what’s going on? How did these “lite” intermediate-mass black holes form? That’s a mystery that will take more observations to solve.