As more powerful observatories have allowed astronomers to peer farther back in time and push the observable edge of the universe closer to the Big Bang, they revealed that most, if not all, galaxies harbor supermassive black holes at their center. Clearly, black holes play a critical role in the formation and evolution of galaxies – but why and how? This question has kept many an astrophysicist and cosmologist up at night.
In a paper published in Nature, a team led by Weizhe Liu and Xiaohui Fan at the University of Arizona’s Steward Observatory reports the discovery of an unprecedented number of exceptionally fast and powerful galactic “winds” streaming from quasars a mere one billion years after the Big Bang (for comparison, the universe is currently 13.8 billion years old). As the supermassive black hole at the center of a quasar galaxy voraciously gobbles up matter, it releases colossal amounts of energy. considered the most energetic objects in the universe, it is not unusual for a quasar to outshine all other light sources in its host galaxy.
The study could hold the key to another cosmological mystery: At very high “redshifts” – the farthest reaches of space and time, in this case within around two billion years after the Big Bang – astronomers had turned up an unexpectedly large number of young galaxies that stopped forming stars early on.
“Many of those galaxies looked ‘old’ in the sense that they had stopped forming stars long before it would be expected,” said Liu, the paper’s first author and a JASPER postdoctoral scholar at Steward Observatory. “How could they have formed so early and become so massive, when they quit star formation so early? That surprising discovery challenged our current paradigm of galaxy evolution, and that was one of the main motivations behind our paper.”
The likely culprits behind this process known as “quenching.” Cosmological simulations suggested that with their blowtorch action, quasars rid their galaxy of its gas supplies, effectively shutting down the process of making stars. Until now, however, nobody had found a smoking gun, as only very few examples of quasars of the right age were known.
JWST observations
Using the James Webb Space Telescope, the U of A-led team scoured the high-redshift universe for quasars and observed 27 such objects from the time of one billion years after the Big Bang. Six of them stood out through exceptionally fast galaxy-scale winds, up to 5,000 miles per second, which Liu said is extremely fast even for a quasar. The survey suggests that quasars with extremely fast outflows were at least four times more common at higher redshifts than at lower redshifts, and their average outflow rate of kinetic energy about 100 times higher compared to lower-redshift quasars.
“In other words, quasars with extreme outflows were much more common in the early universe and became scarcer over time, which is surprising,” said Fan, a Regents Professor and associate head of the Department of Astronomy who is the paper’s second author.
The team believes that such “super quasars” could help explain the abundance of galaxies that stopped forming stars before their time in the early universe. Although a few quasars feature a prominent particle jet, typically shooting out in opposite directions, scientists have long known that it is not the jet that blows the gas out from a galaxy, Fan explained.
“Those jets move at speeds close to the speed of light,” he said. “They essentially just punch a narrow hole into the galaxy. In contrast, the outflows we are talking about here are more like stellar wind, and we think they could be driven in many directions by radiation pressure from the quasar’s extreme bright light.”
The team also estimated that the extreme outflow quasars appear very short-lived, going dormant within about 100 million years – a cosmic blink of an eye – and leaving behind a quiescent galaxy. They estimate that every year, a galaxy with an extreme outflow quasar at its center would lose gas equivalent to thousands of solar masses.
“That is a very high rate of mass loss,” Liu said. “Apply that over the course of – at least – a million years, and you will see you can remove a lot of gas from an entire galaxy over a relatively short period of time.”
The space between galaxies
Because these outflows are very fast, Liu and his team believe they could escape the galaxy and possibly reach into the intergalactic medium – the space between galaxies.
“In other words, these quasars could affect not only their host galaxies, but beyond, with their effects felt possibly hundreds of thousands of light-years away,” Liu said.
The observations of extreme outflows provide a missing link connecting supermassive black holes to galaxies, according to Fan.
“These winds are a direct result of the black hole growing by accreting mass, and later, and when the black hole stops growing, the winds subside as well,” he said. “So we’re seeing the direct interactions between the black holes and the galaxies in which they reside, during at early time of the universe.”
The findings also offer a possible explanation as to how galaxies can quit star formation so early on in the universe. Early galaxies were likely more compact, “gassier” and “clumpier” than their more evolved, spiral counterparts today. This has important implications for the interaction between a galaxy and its supermassive black hole, according to Liu.
“It is much easier for a quasar to interact with the gas around it if the gas is denser and distributed all around it rather than confined to a thin disk,” Liu said. Combined with the higher outflow rates from an extremely fast-accreting quasar, the galaxies’ structural makeup would have made it even easier for a quasar to rid its galaxy of more gas more rapidly and more effectively.
“In short, the impact of the black holes on their host galaxies through this process would have been more effective than in an older, more evolved galaxy in the later universe,” he said.