Black holes massacre galaxies: astronomers uncover the gory details

Black holes are known to be silent killers of galaxies, but new discoveries made by the James Webb telescope have shed light on how they go about it.

Using the James Webb Telescope, NASA’s and ESA’s largest telescope in space, an international team of astronomers has made discoveries that confirm the theory that black holes can ‘starve’ their host galaxies to death.

"We found the culprit," said co-lead author Dr. Francesco D'Eugenio from Cambridge's Kavli Institute for Cosmology. "The black hole is killing this galaxy and keeping it dormant, by cutting off the source of 'food' the galaxy needs to form new stars."

The dead galaxy

Large galaxies, including our own Milky Way, contain supermassive black holes at their centers, known for their capacity to absorb matter. When a black hole actively consumes matter, it’s referred to as being in the quasar phase, which could last a few million years. The intense radiation emitted by quasars affects the star formation in galaxies.

It can heat the surrounding gas to such high temperatures that it can't cool down enough to collapse and form new stars, slowing down the galaxy's growth.

To dive deeper into the universe's secrets, astronomers, co-led by the University of Cambridge, investigated a galaxy nicknamed “Pablo’s Galaxy” in the early stage of the universe, about two billion years after the Big Bang.

While it’s named after the researcher who decided to study it, the celestial system is officially called GS-10578, and like most big galaxies, it’s basically ‘dead.’ A galaxy is pronounced dead when it stops forming new stars, and Pablo’s stars were mostly formed between 12.5 and 11.5 billion years ago.

This is a fascinating phenomenon because, in the early universe, most galaxies were actively forming stars. A massive, dead galaxy from this period is therefore unusual. The process that halted star formation must have occurred relatively quickly. Given that a giant black hole sits at its center, it likely played a significant role in the galaxy's demise.

"Based on earlier observations, we knew this galaxy was in a quenched state: it's not forming many stars given its size, and we expect there is a link between the black hole and the end of star formation," said D'Eugenio “However, until Webb, we haven't been able to study this galaxy in enough detail to confirm that link.”

The evidence leads to the killer

Using the Webb telescope, the researchers detected that Pablo’s galaxy is expelling large amounts of gas at speeds of about 1,000 kilometers per second. The black hole in the center is the likely culprit, pushing these fast-moving winds out of the galaxy.

In a report in the journal Nature Astronomy, scientists claim that hot gas clouds are tenuous and have little mass. The Webb telescope also allows researchers to detect a new gas wind component that could not be seen with earlier telescopes.

This gas is colder, which means it's denser and does not emit any light. The mass of gas being ejected from the galaxy is greater than what the galaxy would require to keep forming new stars. This is proof that the black hole is ‘starving’ the galaxy to death.

Previous theories suggested that the end of star formation would have a violent, turbulent impact on galaxies, disrupting their structure. However, the stars in this disk-shaped galaxy are still moving in an orderly fashion despite being killed.

"We knew that black holes have a massive impact on galaxies, and perhaps it's common that they stop star formation, but until Webb, we weren't able to directly confirm this," said co-author Professor Roberto Maiolino.

"It's yet another way that Webb is such a giant leap forward in terms of our ability to study the early universe and how it evolved."