&Bullet; physics 14, 67
The latest data set from gravitational wave observatories contains enough events to allow researchers to study the properties of the entire black hole population.
Less than six years after gravitational waves were first detected, observations are becoming routine. LIGO and Virgo log black hole fusions more than once a week. At the APS meeting in April, the LIGO-Virgo Collaboration (LVC) reported that it assessed the typical characteristics and stories of black holes based on its catalog of almost 50 events. For example, measurements of black hole spins suggest that at least two different formation mechanisms are common for black hole binaries. These “population” studies of the black hole – similar to astronomers’ star studies – are becoming a valuable tool for gravitational wave scientists along with studies of individual events.
The black holes in the LVC catalog are stellar-mass black holes – the remains of giant stars after they exploded as supernovae. In the past, astronomers could only see these black holes if they were in a binary orbit with a normal star. However, the LIGO and Virgo observatories have revolutionized the field since 2015. “The vast majority of the black holes known to us with stellar mass, for example, in the universe [were detected via] Gravitational waves, ”said Carl Rodriguez of Carnegie Mellon University, Pennsylvania, in his talk at the conference. Gravitational waves are now the main source of data from which astrophysicists will learn about these objects. “For the first time we can do astronomy,” said Maya Fishbach of Northwestern University, Illinois, a member of the LVC. “We’re really going to learn more about star formation in general.”
In his lecture, LVC member Thomas Dent from the University of Santiago de Compostela in Spain presented several results based on analyzes of 47 events. He showed the mass distribution – a graph of the rate of fusion versus the primary (larger) mass of a merging black hole pair. The rate decreases with increasing mass, and the curve can be mathematically described as having a small bump at about 33 solar masses, although the shape is not yet entirely certain. This feature reflects the fact that there is an excess of fusions with primary masses in the range of about 30 to 40 solar masses and then a sudden decline at higher masses.
These properties of the mass spectrum are not very well understood, Dent said. A pair instability supernova (PISN) – a type associated with the largest stars – is expected to leave behind either a small black hole or no black hole at all, creating a gap in the spectrum between about 45 and 120 solar masses. However, the latest catalog, which contains data through September 2019, contains several black holes weighing up to 85 solar masses. In addition, a “cluster” of black holes just under 45 solar masses was also expected, resulting from a process in which stars at the lower end of the PISN range lose mass and produce more typical supernovae. However, the observed elevation is 33 solar masses, which is too low to conform to conventional theory. The models could be tweaked a bit, Dent said, but there is currently no model that can accommodate both the low-mass bump and the higher-mass black holes in the predicted void.
Dent also discussed the results of the collaboration on black hole spins and what they mean for the history of black hole binaries. Researchers frequently discuss two types of binary black hole formation mechanisms: isolated and dynamic. Isolated black hole binaries are formed from binary star systems in which both stars die and black holes leave behind. Dynamic binaries are formed in star-rich environments like pebbles when two individual black holes meet and become gravitationally bound.
In isolated binaries, it is expected that the spin axes of the two black holes are relatively well aligned with the orbital axis that defines their movement around each other, and that their spins are in the same direction as their orbits (clockwise or counterclockwise). However, the axes and directions of rotation of a dynamic couple should not be correlated, as they were rotating long before they met. The latest catalog data shows a wide distribution of spin-orbital alignment, including between 12 and 44% of events where one or both spins are directed against their orbital motion. If the spins were completely uncorrelated, this proportion would be 50%. The fact that the antialigned faction is significant – but less than half – suggests that both educational mechanisms must be at work, Dent said.
David Ehrenstein is Senior Editor for physics.