This is as close to a smoking gun as we’re likely ever going to get, Yale University’s Pieter van Dokkum announced, after his team made their find in the Infinity galaxy a cosmic feature whose existence is rewriting the story of how supermassive black holes may be formed.
The Infinity galaxy, about 8 billion light-years away, originally caught scientists’ notice with its unique morphology: two bright, dense nuclei, each surrounded by a ring of stars, jointly looking like the infinity symbol. This unusual formation, as van Dokkum’s team discovered, probably resulted from a head-on collision between two disk galaxies, a process that dramatically remolded both the stellar and gaseous materials.
But the biggest surprise was not in the shape of the galaxy, but in its center. Betwixt the two nuclei, in a wide sweep of ionized hydrogen, lies a supermassive black hole actively accreting but not tied to either galactic core. “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?”
van Dokkum explained to NASA. This unexpected placement defies conventional theories of black hole origin. Historically, astronomers have argued two broad avenues: the “light seed” model in which black holes are produced by the collapse of heavy stars and grow over time via mergers and accretion, and the “heavy seed” or direct collapse model in which a large gas cloud collapses directly into a black hole without going through the stellar stage.
The heavy seed hypothesis, while much established, fails to account for the presence of supermassive black holes that were observed less than a billion years after the Big Bang, since the growth time needed is just too long. The Infinity galaxy provides a unique, possibly unprecedented, laboratory for the heavy seed hypothesis. When the two galaxies collided, their interstellar gas was compressed and shocked, producing conditions extreme enough to probably trigger the direct collapse of a dense knot of gas into a black hole.
Simulations of such an occurrence, particularly mergers at high redshift, have indicated that gravitationally bound disks with masses of several billion suns can create themselves in under 100,000 years, paving the way for quick black hole creation. In order to exclude other explanations like a runaway black hole that has been expelled from another location, or a random alignment with a third, less luminous galaxy the group used velocity measurements. With the James Webb Space Telescope’s NIRSpec instrument, they matched the ionized gas and black hole velocities.
The findings were dramatic: the velocity of the black hole was the same as the surrounding gas to within 50 km/s, a strong signal that it shared a common origin with that gas, instead of being an interloper.
Multiwavelength observations proved key. Chandra X-ray observations demonstrated the black hole’s energetic accretion, and radio imaging by the Very Large Array verified it as an active black hole. Both galactic nuclei have supermassive black holes of their own, setting this system apart as a triple-black-hole system a genuine astrophysical oddity in its own right. The consequences have far-reaching implications beyond a single galaxy. The Infinity galaxy’s black hole could be a contemporary parallel to the mechanisms that initiated the universe’s first quasars. Direct collapse theories propose the suppression of cooling via molecular hydrogen is necessary, usually attained via extreme ultraviolet backgrounds or dynamical shocks, so as not to initiate premature star formation and permit the gas to collapse monolithically.
The demonstrated compression and shock heating in the collision of the Infinity galaxy fulfill these conditions, offering a terrestrial testbed for simulation predictions.
However, as has been demonstrated in recent attempts to use JWST to search for direct-collapse black hole candidates, it continues to be difficult to differentiate these from dusty galaxies or obscured active galactic nuclei. Spectroscopic follow-up, searching for strong hydrogen Balmer emission and a lack of metal lines, could provide the ultimate signature. For now, the Infinity galaxy stands as the most compelling evidence yet for the direct collapse pathway, suggesting that the universe may have multiple routes to birthing its most enigmatic giants. As van Dokkum put it, “We think we’re witnessing the birth of a supermassive black hole something that has never been seen before.”
