According to the current cosmological paradigm, the universe is a living ecosystem in which gases flow into and out of galaxies driven by the massive black holes found at their centres. But how much impact do these massive black holes have on the evolution of the galaxy itself?
This was the question asked – and answered – by the FEEDBACK project, which was funded specifically by the European Research Council (ERC).
The power of the black hole
The project focused its research on active galaxy nucleus (AGN) feedback in galaxies and galaxy clusters. “An AGN is characterised by a luminous and powerful accreting black hole at the centre of the galaxy,” says Andrew Fabian, an astronomer at the University of Cambridge and FEEDBACK principal investigator. “Feedback, on the other hand, is how energy coming from a black hole can alter a galaxy that is a billion times larger than the black hole itself.”
According to Fabian, there are several possible ways that feedback can modify a galaxy and its surroundings. On the one hand, by ejecting gas, feedback can prevent further stars from forming, thus essentially killing the galaxy. Outflowing gas can also stimulate the formation of new stars and, in doing so, change the galaxy’s shape. Lastly, if the galaxy is part of a galaxy cluster, the gas can become heated.
“We tackled issues concerning the generation of power in a black hole’s accretion flow,” explains Fabian. “This meant studying not only the geometry of the flow, mass and spin of the black hole and its energy production processes, but also how the energy impacts its surroundings – particularly in the case of galaxy clusters.”
In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter. Galaxies, stars, planets and other astronomical objects are formed via accretion.
Satisfying our curiosity
Much of FEEDBACK’s research was observational, conducted using X-ray telescopes combined with simple theory and modelling. For example, in February 2016, the Hitomi satellite made deep X-ray observations of the cool core of the Perseus cluster of galaxies. “The resulting spectra were of high resolution – unprecedented in cosmic X-ray astronomy,” adds Fabian.
Other modes of observation included the NuSTAR group of orbiting telescopes and NASA’s neutron star interior composition explorer (NICER) telescope, which is located on the International Space Station (ISS).
From these observations, researchers found that AGN feedback plays a major role in a galaxy’s evolution – a finding that greatly expands our understanding of the universe. “Seeing how exotic objects like black holes can influence galaxies satisfies our curiosity about the origin of the largest structures in the universe,” remarks Fabian.
Fabian’s work on the FEEDBACK project helped him earn the Norwegian Academy of Science and Letters’ 2020 Kavli Prize for Astrophysics. “Andrew Fabian, one of the most prolific and influential astronomers of our time, has been a leading figure in the field of observational X-ray astronomy, covering a wide range of topics from gas flows in clusters of galaxies to supermassive black holes at the heart of galaxies,” reads an Academy announcement. “Fabian’s breadth of knowledge and insights on vastly different scales have provided key physical understandings of how those disparate phenomena are interconnected.”
Fabian accredits some of his success to the EU funding he received from the ERC, which allowed him to assemble and retain a team of productive researchers for over 5 years. “The funding flexibility was of great benefit both for hiring and for travelling to meetings,” he adds. “It also meant the team could concentrate on doing excellent science.”
Six postdocs from the FEEDBACK team have since secured full-time faculty positions, while two others have fellowships. All continue to work on unravelling the mysteries of the universe.