Astronomers have added four previously overlooked black hole mergers to the list of detections, bringing the total to 10. One of these is the largest, and most distant, we have yet seen, with the merged black hole having a mass 80 times that of the Sun.
The first long-sought detection of a black hole merger in 2016 was among this decade's biggest physics advances. However, Professor Susan Scott of the Australian National University told IFLScience that to really advance our knowledge of these grand events we need a sample of mergers large enough to reveal patterns.
To help achieve this merger population, Scott was part of a team that dug through the data produced by the Advanced Laser Interferometer Gravitational-Wave Observatory's (LIGO) first two observing periods to seek events that had previously been missed. The four events found were reported in Physical Review X.
“We characterize black holes purely by mass and spin, unlike stars that have many other characteristics,” Scott told IFLScience. These two criteria, along with their distance from Earth, provide the only numeric on which to measure black hole mergers. An event on July 29, 2017, is the most extreme we've observed on all three.
Besides producing the black hole with the largest combined mass, Scott said in a statement that “this event also had black holes spinning the fastest of all mergers observed so far.” At 9 billion light-years away, Scott noted that “it is also by far the most distant merger observed.”
It is thought all observed events are the result of mergers between black holes produced when very large stars collapse, such as in a Type II supernova. One possibility researchers are interested in is the idea that some of what we are witnessing represents second-generation mergers of already merged holes among stellar remnants in tightly packed clusters. One of the reasons Scott wants larger sample sizes is the possibility of identifying a distinctive subgroup of larger holes formed by such sequential mergers.
Scott told IFLScience the supermassive black holes at the centers of galaxies are “sucking in everything in the nearby vicinity”. This presumably includes smaller black holes, but we haven't detected these events. She explained this is because the larger the black holes, the shorter the signal. The need to exclude short-term random noise puts an upper limit on the size of the merged objects we can detect with existing equipment.
At the other end of the scale, possibly the most important event LIGO has witnessed was the merger between two neutron stars. Although this event lasted longer, the signals neutron stars' relative lightness creates are weaker, reducing the volume of space over which we can find such events, and Scott's team didn't find any more.