In order for galaxies to grow, they need raw material for star formation – cool, neutral gas. But now astronomers have discovered an early galaxy from which this raw material is being extracted. The James Webb Space Telescope revealed that in this galaxy, large amounts of neutral gas are being ejected outwards – most likely by the galaxy’s central black hole. According to the astronomers, this explains why the galaxy, named GS-10578, has stagnated in its growth and is hardly forming any stars anymore despite its relatively young age: the outflow of gas causes it to lose the raw material for star formation. This confirms a long-standing theory about this phenomenon of so-called “quenching” in early galaxies: the early period of rapid star formation was prevented or at least temporarily halted by the activity of their central black holes.
Most galaxies experienced a period of rapid star formation in their early days – they formed thousands of new stars per year. Such a starburst phase can also be observed in many early galaxies that are billions of light years away and which we therefore see as they looked at the beginning of cosmic development. Today’s galaxies, on the other hand, are comparatively unproductive. In our Milky Way, for example, only a handful of new stars are formed each year. “The local, massive galaxies appear like colossal wrecks of a glorious but distant history of star formation, which was followed by a powerful, rapid quenching,” explain Francesco D’Eugenio from the Kavli Institute for Cosmology at the University of Cambridge and his colleagues. Quenching is what astronomers call the sudden, sharp decline in star formation.
Early galaxy in hibernation
Many galaxies experienced such quenching just a few billion years after the Big Bang. “Such massive quiescent galaxies have little to no cold gas left, the raw material for star formation,” the astronomers explain. Galaxy growth then stagnates unless it is fueled again by a galaxy merger. However, it has not yet been clear what stops star formation in such quiescent galaxies. The previous phase of overactive star formation could simply have consumed all the raw material, but it is also conceivable that the central black hole of these galaxies plays a role – for example by blowing the cold, neutral gas out of the galaxy. “The exact mechanism of this quenching is not understood because the local massive galaxies experienced this billions of years ago,” the astronomers say.
But now D’Eugenio and his team have succeeded in tracking down an early galaxy in which this quenching is only just beginning. Using the James Webb telescope, the astronomers studied the distant, almost quiescent galaxy GS-10578, which existed around two billion years after the Big Bang and was already about the size of the Milky Way at that time. “Based on previous observations, we knew that this galaxy was in a quenched state: it hardly forms any stars anymore compared to its size,” reports D’Eugenio. “But before Webb, we were unable to study this galaxy in sufficient detail.” The new observations now confirm that star formation in GS-10578 has indeed almost come to a standstill. It only forms around 19 solar masses of new stars per year, the team determined. That is less than a fifth of what was normal for normal galaxies of this size at that time.
Black hole pushes gas outwards
The images from the Webb telescope also revealed a possible cause for this halt in star formation: in addition to the normal outflow of ionized gases that occur in many galaxies, the astronomers discovered a strong outflow of cool, neutral gases from this galaxy. This transported up to 100 solar masses of gas outwards and into space at high speed every year. “This outflow of neutral gas is enough to stop star formation by depriving it of the necessary raw material,” write D’Eugenio and his colleagues. They identified the central black hole of the galaxy as the cause of this gas loss. Its activity drives the gas outwards: “GS-10578 is home to an active galactic nucleus and the outflows are due to the feedback from this supermassive black hole,” explain the astronomers. “Although we cannot draw general conclusions from just one target, this shows that the effects of an active galactic nucleus are capable of causing neutral gas outflows of high mass and high velocity – enough to disrupt star formation.”
The observations in the distant galaxy GS-10578 prove that a central black hole can actually “starve” its host galaxy. “We knew that black holes have a big influence on their galaxies and it is probably quite common for them to prevent their star formation. But it was only with the Webb telescope that we were able to confirm this,” says co-author Roberto Maiolino from the University of Cambridge. The astronomers next want to target the galaxy GS-10578 with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile in order to be able to use its radio telescopes to observe the coolest, darkest gas parts of the galaxy.
Source: Francesco D’Eugenio (Kavli Institute for Cosmology, University of Cambridge) et al., Nature Astronomy, doi: 10.1038/s41550-024-02345-1