At the center of almost every galaxy lies a supermassive black hole. Some are dramatically active, others are quiet, but all of them play a crucial role in the evolution of galaxies. Figuring out how these black holes grow is important in understanding how other structures in the Universe have changed since the Big Bang.
Black holes can grow in two ways: they either merge with other black holes or they gobble up material that assembles around them. To investigate the growth of black holes further, researchers created a theoretical model that suggests the growth of a black hole depends on its mass and the period of the universe in consideration. In the early universe, small black holes grew mostly from mergers with other black holes, while bigger black holes gained mass by accretion.
"We believe that every galaxy contains a massive black hole at its center, which regulates the formation of stars in their host,” lead author Dr Fabio Pacucci of Harvard University said in a statement. "Understanding how black holes formed, grew, and co-evolved with galaxies is fundamental to our understanding and knowledge of the universe, and with this study, we have one more piece of the puzzle."
The paper testing this theoretical framework is straightforward. Black holes growing from the accretion of matter are expected to spin on their axis faster than those that grow through mergers. However, current technological limitations make extensively testing the claims difficult.
"Understanding how black holes grow as a function of their mass and of the cosmic time is important, first and foremost, because the growth channel determines which type of observations are better suited for a specific black hole," Pacucci told IFLScience. "Gravitational wave observations probe mergers, while electromagnetic observations (in the infrared and in the X-rays, for example) probe accretion. Hence, our model will guide future observational efforts to detect ancient populations of black holes. Mapping the broad population of black holes out in the early Universe, in turn, will be instrumental in providing a clear picture of how black holes grew and co-evolved with galaxies across cosmic time."
Future X-ray and gravitational wave observatories such as Athena and LISA will be able to create a larger catalog of black hole spins across the ages of the universe. This will be a crucial test for a model such as this. The study is published in The Astrophysical Journal and was presented at the 236th meeting of the American Astronomical Society.