What if the universe’s most mysterious monsters aren’t born in darkness, but in the bright turbulence of colliding galaxies? That question now excites astrophysics, as the James Webb Space Telescope (JWST) provides what is possibly the first direct look at a supermassive black hole (SMBH) being formed not at the center of a galaxy, but in the chaotic no-man’s-land between two colliding giants.
The Infinity Galaxy, so named for its striking resemblance to the infinity symbol, sits 8.3 billion light-years away, its light now reaching JWST’s sensors after a journey that began when the universe was just over half its current age. In the heart of this cosmic collision, astronomers have detected a newborn SMBH, not nestled within either galactic nucleus, but suspended in a vast, shocked cloud of gas between them. “Everything is unusual about this galaxy. Not only does it look very strange, but it also has this supermassive black hole that’s pulling a lot of material in. The biggest surprise of all was that the black hole was not located inside either of the two nuclei but in the middle. We asked ourselves: How can we make sense of this?” said Pieter van Dokkum, senior author of the find and professor at Yale University in a peer-reviewed study. This discovery goes to the heart of a long-standing question: how do supermassive black holes million to billion times the Sun’s mass form so quickly after the Big Bang? The “light seeds” hypothesis suggests that SMBHs grow gradually through the coalescence of small black holes, which themselves are the remnants of super-massive stars. However, it is a gradual process, and does not so readily explain the existence of huge black holes already seen less than a billion years after the Big Bang by JWST and other observatories.
The second, the “heavy seeds” or the direct-collapse model, proposes that, under specific circumstances, gigantic clouds of gas can avoid star formation and directly collapse into a black hole, creating a huge seed in a fraction of the time. The Infinity Galaxy provides strong evidence for this latter hypothesis. JWST’s NIRCam and MIRI instruments, optimised for high-sensitivity near- and mid-infrared imaging and spectroscopy, allowed astronomers to look past the dust and gas and find a strip of ionized hydrogen hydrogen that has been stripped of electrons around the central SMBH candidate. Further observations using the Keck Observatory, Chandra X-ray Observatory, and the Very Large Array (VLA) verified that this object is actively accreting matter, as indicated by the presence of both X-ray and radio radiation. “We observe a large swath of ionized gas, specifically hydrogen that has been stripped of its electrons, that’s right in the middle between the two nuclei, surrounding the supermassive black hole. We also know that the black hole is actively growing – we see evidence of that in X-rays from NASA’s Chandra X-ray Observatory and radio from the Very Large Array,” van Dokkum said in his team’s report.
Most importantly, the velocity of the gas is identical to that of the black hole, indicating a shared origin. “The black hole is beautifully in the middle of the velocity distribution of this surrounding gas – as expected if it formed there. This is the key result that we were after!” van Dokkum explained. This alignment actually eliminates competing explanations, including a runaway black hole from a distant galaxy or an unseen dwarf galaxy containing the black hole. Hydrodynamical models have for a long time predicted that significant mergers of gas-rich, disk-dominated galaxies should power multiscale gas inflows toward the center of the merger remnant, forming compact, gravitationally bound disks of up to a billion solar masses in less than 100,000 years as described in recent simulation work.
These disks, stabilized against fragmentation by high accretion rates and turbulent flows, can directly collapse into supermassive stars or protostars, which form black holes quickly. The Infinity Galaxy, filled with dense, turbulent gas and devoid of a pre-existing central black hole at the site of collision, replicates these simulation conditions with remarkable accuracy. The combination of JWST’s record infrared sensitivity and the multi-wavelength capability of Keck, Chandra, and VLA has provided a systems-level understanding of this system.
Observations of three active SMBHs, one in every galactic nucleus and one in the intergalactic medium, highlight the richness of black hole formation and evolution during galaxy collisions. “This system has three confirmed active black holes: two very massive ones in both of the galaxy nuclei, and the one in between them that might have formed there,” van Dokkum said. As conclusive evidence of direct-collapse continues to elude scientists, the Infinity Galaxy presents the strongest empirical evidence to date that SMBHs can directly emerge from collapsing gas clouds following galaxy collisions. As JWST keeps venturing deeper into the history of the cosmos, astronomers expect more such systems to be revealed, with the potential to rewrite the book on how the universe’s largest black holes form. As van Dokkum and Brammer concluded, “If our proposed scenario is confirmed, the Infinity galaxy provides an empirical demonstration that direct-collapse formation of SMBHs can happen in the right circumstances something that has so far only been seen in simulations and through indirect observations.”
