When the James Webb Space Telescope launched on Christmas Day 2021, it faced a whole host of intriguing questions. When it finally launched, astronomers had a long list of targets that demanded the kind of detailed observations that only the powerful infrared space telescope could make. One of the targets was an old, massive galaxy that is essentially dead and not forming new stars.
The results are in and an international team of astronomers knows what happened to the dormant galaxy.
The growth and evolution of galaxies is a central area of research in astronomy. How did we get from the Big Bang to today, when massive galaxies like our own Milky Way populate the universe? Astronomers have discovered that supermassive black holes (SMBHs) lie at the heart of massive galaxies and have shaped these galaxies in powerful ways.
SMBHs create powerful active galactic nuclei (AGN) in the cores of galaxies. When an SMBH attracts material, the material accumulates in an accretion disk. The material is heated to extremely high temperatures and releases energy across the entire electromagnetic spectrum, creating an AGN that can outshine the rest of the galaxy.
AGN are powerful objects. Theory suggests that they have the power to cut off the supply of cold star-forming gas and dramatically slow the star formation rate (SFR) in their host galaxy. They blow winds of star-forming gas out of their galaxies, which slows the SFR. Astronomers call this quenching, and it is commonly observed in massive galaxies, known as quiescent galaxies.
Now, the JWST has observed an old, massive galaxy called GS-10578 at a redshift of z?=?3.064. Nicknamed “Pablo's Galaxy,” it is quite massive for such an early stage in the evolution of the Universe: it has about two billion solar masses. But Pablo's Galaxy is annihilated, meaning that most of its star formation processes occurred between 12.5 and 11.5 billion years ago. Many local massive galaxies are annihilated, which has spurred the development of the AGN extinction theory.
A team of scientists has presented his research on Pablo's Galaxy in a new paper titled “A rapidly rotating post-starburst galaxy quenched by the feedback of a supermassive black hole at z?=?3”. The paper is published in Nature Astronomy, and the co-lead author is Francesco D'Eugenio of the Kavli Institute for Cosmology and the Cavendish Laboratory at the University of Cambridge in the UK.
“We have found the culprit. The black hole is destroying this galaxy and keeping it inactive by cutting off the food source it needs to form new stars.”
Francesco D'Eugenio, Kavli Institute of Cosmology, University of Cambridge, UK
“Local, massive, dormant galaxies stand like colossal wrecks of glorious but distant star formation histories (SFHs) and massive and rapid extinctions, the likes of which are unparalleled today,” the authors write. “The James Webb Space Telescope (JWST) has enabled us for the first time to observe these monumental galaxies during the long-ago epoch in which they formed and died.”
“Based on previous observations, we knew that this galaxy was in an annihilated state: given its size, not many stars are forming, and we assume there is a connection between the black hole and the end of star formation,” said co-lead author Dr. Francesco D'Eugenio of the Kavli Institute for Cosmology in Cambridge. “However, until Webb, we had not been able to study this galaxy in enough detail to confirm this connection, and we did not know whether this annihilated state was temporary or permanent.”
“In the early Universe, most galaxies form many stars, so it is interesting to see such a massive dead galaxy at this time,” said co-author Professor Roberto Maiolino, also of the Kavli Institute for Cosmology. “If it had enough time to reach this enormous size, the process that stopped star formation probably happened relatively quickly.”
Pablo's galaxy is sometimes called a “blue nugget,” a class of galaxies thought to have existed only in the early universe. Blue nuggets are massive and extremely compact, and astronomers believe they are precursors to modern quiescent galaxies called “red nuggets.” Blue nuggets experience “gas-rich compaction.” This means there is a central burst of star formation caused by disk instability or gas-rich large mergers. This burst is followed by an annihilation, leaving behind a red nugget galaxy.
Artist's impression of a “Red Nugget” galaxy. Image credit: X-ray: NASA/CXC/MTA-Eötvös University/N. Werner et al., Illustration: NASA/CXC/M. Weiss
“In fact, as we will show, GS-10578 is already a red nugget in an advanced stage of quenching,” the authors write. They explain that it is merging with several low-mass satellite galaxies and “is experiencing a strong ejective feedback from its SMBH.”
The researchers say they have direct evidence that AGN feedback can shut down star formation in early galaxies. Previous observations with other telescopes show that galaxies have rapidly outflowing gas winds. This gas is hot, making it easier to detect, but did not provide evidence that SMBHs and AGN can shut down star formation. That's because the gas is hot, and stars form from cold, dense gas.
Pablo's galaxy is no exception. It is ejecting large amounts of hot gas at a speed high enough to completely exit the galaxy. The SMBH and its AGN are pushing the gas out.
But the JWST made the difference in these new observations. It observed a new component of the outflowing wind, made up of cold gas. The cold gas does not emit light, but the JWST is extremely sensitive and can detect it by blocking the light from distant galaxies in the background. Crucially, a galaxy without cold gas will struggle to form stars and will die out.
This image illustrates some of the research. In the center is Pablo's Galaxy, where five low-mass satellite galaxies are merging. The inset (b) shows a section of the main image. The cyan outline is offset to the northwest and represents the outflow of cold gas that is inhibiting star formation in the galaxy. Image credit: D'Eugenio and Maiolino et al. 2024.
The amount of gas ejected by AGN-driven winds is greater than the amount required to form new stars.
“We have found the culprit,” said D'Eugenio. “The black hole is destroying this galaxy and keeping it inactive by cutting off the 'food source' it needs to form new stars.”
These are exciting results, but the authors point out that it's just a single galaxy. “GS-10578 represents a unique opportunity to study how the most massive galaxies in the Universe became – and remained – inactive,” the authors explain in their research. “While we cannot draw general conclusions from a single target, we show that AGN feedback can drive neutral gas outflows at high speeds and high mass loadings, sufficient to shut down star formation by removing their cold gas fuel.”
There are also unanswered questions. Other galaxies similar to Pablo's also show that outflow winds of cold gas may be key to quenching galaxies. “How exactly these outflows are coupled to the AGN is not yet clear,” the authors write. They explain that only an inventory of similar galaxies can tell us whether these strong outflows of star-forming gas are a key mechanism for quenching, or whether the ejection of gas is merely episodic.
JWST also answered another open question about annihilated galaxies. Our theoretical models showed that the annihilation of star formation in a galaxy was a turbulent event that violently disrupted the galaxy's shape. Pablo's galaxy still has the stately disk shape of an undisturbed galaxy. Its stars move in a smooth, predictable manner.
This image from the study shows the orderly rotation of the Pablo Galaxy. The observed difference in speed is due to one side moving away from us and being redshifted from our perspective, while the other is moving toward us and being blueshifted. Image credit: D'Eugenio and Maiolino et al. 2024.
The JWST is working exactly as intended. By making the early universe visible, it is answering many long-standing questions in astronomy, astrophysics and cosmology.
“We knew that black holes have a massive impact on galaxies and that it might be common for them to stop star formation, but until Webb we couldn't confirm that directly,” Maiolino said. “That's another reason why Webb is a huge step forward in our ability to study the early universe and its evolution.”
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