Not often do astronomers come across a tear in the tissue of a galaxy a scab so big it might span one-fifth of the Milky Way. But in the spiral galaxy Leo’s NGC 3627, scientists have discovered just that: a 20,000-light-year-long trail of gas and dust, so thin only 650 light-years wide that it appears nearly surgical in its precision. The offender, they think, is a titanic black hole that ripped through the galactic disk at unprecedented velocities.
The finding came from the Physics at High Angular Resolution of Nearby Galaxies (PHANGS) survey, a multi-telescope effort marrying the clear infrared eyesight of the James Webb Space Telescope (JWST) with the molecular gas mapping ability of the Atacama Large Millimeter/submillimeter Array (ALMA). JWST’s observations showed the contrail’s dust content and ALMA observed rich carbon monoxide, a cold molecular gas tracer. This double signature indicates that the trail is more than an optical illusion but rather a real displacement of interstellar material.
Theoretical foundations for such a process are provided by a 2021 model developed by Guang-Xing Li, whereby a giant compact object moving through a galactic disk compresses and pushes gas, creating a thin, turbulent contrail. In NGC 3627, the turbulence in the contrail is as the model predicts. Mathematics suggests an object weighing around 10 million solar masses, traveling at 300 kilometers per second about 50 percent faster than the record set by the Parker Solar Probe. The encounter is estimated to have occurred roughly 20 million years ago, a blink of an eye in cosmic time.
The intruder’s nature is not clear. “Currently, with the available data, we cannot definitively distinguish between these two possibilities,” said Mengke Zhao of Nanjing University, whether the object is a supermassive black hole or the compact nucleus of a feeble dwarf galaxy. Deep optical surveys or ultra-high resolution ALMA imaging are needed to detect such a feeble dwarf galaxy at the distance of NGC 3627.
A third explanation brings mystery: the mysterious “little red dots” that appear in JWST’s deep-field views. These small, intensely red objects that emit mostly in the mid-infrared have perplexed astronomers since they were first detected in 2022. One theoretical explanation, buttressed by recent observations of an object dubbed The Cliff, is that they are “black hole stars” supermassive black holes encased in thick hydrogen envelopes, whose chaotic atmospheres reproduce some stellar spectral features. If such an object was traveling through NGC 3627, its gravitational effect could be responsible for the described contrail.
It takes consideration of galactic dynamics to appreciate how a black hole or black hole star may travel through a galactic disk at such a speed. Compact objects can be launched by gravitational interactions, like the merger of two galaxies or ejection of a black hole after asymmetric emission of gravitational waves during a merger. Once they get moving, their massive gravity can strip gas off the disk, squishing it into a thin wake. The dynamics are similar to a supersonic plane creating a shockwave, except in this case the medium is interstellar gas and the velocities are hundreds of kilometers per second.
If the intruder is a dwarf galaxy nucleus, its presence would draw on methods employed to detect faint galaxies in distant systems. Astronomers use deep imaging and spectroscopy to distinguish low-luminosity objects against the high-contrast background of their host galaxies. In this instance, ALMA’s sensitivity to molecular gas might detect faint gravitational perturbations characteristic of a compact companion.
The contrail’s chemical makeup dust and carbon monoxide also provides hints. Dust particles survive in thick molecular clouds but are quickly destroyed in warm, ionized gas. The presence of such particles indicates the wake has been quite cool, despite the turbulence, retaining molecular gas that may eventually be compressed into new stars. But if the wake is the result of a black hole producing energetic jets, as observed in high-energy quasars, those jets can quench star formation by heating or dispersing the gas.
Zhao and Li intend to dig through PHANGS data for more such tracks, and hope to decide if NGC 3627’s gash is an isolated event or belongs to a larger population of dark mass objects sailing through galactic disks. “Understanding their evolution and how common they are could tell us a lot about the population of massive dark objects flying through galactic disks,” Zhao said. Each finding would not merely deepen the enigma of these galactic interlopers but also sharpen models of how galaxies evolve under the rule of their most violent residents.
