Unprecedented Cosmic Menace: Escaped Supermassive Black Hole Defies All Expectations

A Bizarre 200,000-Light-Year-Long Bridge Links a Galaxy to Its Escaping Black Hole

Yale astronomer Pieter van Dokkum experienced a striking example of the universe’s capricious nature. While examining images from the Hubble Space Telescope, he made an intriguing observation: a suspected flaw resembling a scratch on photographic film. In the context of Hubble’s electronic cameras, such marks are often attributed to cosmic rays gliding along the detector, appearing like scratches. However, upon subjecting the peculiar streak to spectroscopy, van Dokkum made a remarkable realization. It turned out to be a chain of young blue stars extending an astonishing distance of 200,000 light-years, positioned beyond the halfway mark across the universe. The astronomers propose that this elongated chain connects a runaway supermassive black hole to the galaxy from which it was forcefully ejected. Evidently, the black hole’s passage causes the compression of gas in its wake, leading to the formation of stars. This extraordinary phenomenon represents an unprecedented occurrence, never before observed anywhere else in the cosmos.

This is an artist’s impression of a runaway supermassive black hole that was ejected from its host galaxy as a result of a tussle between it and two other black holes. As the black hole plows through intergalactic space it compresses tenuous gas in front to it. This precipitates the birth of hot blue stars. This illustration is based on Hubble Space Telescope observations of a 200,000-light-year-long “contrail” of stars behind an escaping black hole. Credit: NASA, ESA, Leah Hustak (STScI)

Hubble Space Telescope Sees Possible Runaway Black Hole Creating a Trail of Stars

In the vast expanse of intergalactic space, an elusive and unseen behemoth roams freely, traversing at such immense speed that it could journey from Earth to the Moon within a mere 14 minutes if it were present in our solar system. This supermassive black hole, boasting a mass equivalent to 20 million Suns, has left behind an unprecedented “contrail” of newborn stars, stretching an astonishing length of 200,000 light-years, twice the diameter of our own Milky Way galaxy. The origins of this phenomenon can be attributed to a peculiar and rare cosmic interaction involving three massive black holes, akin to a perplexing game of galactic billiards.

Rather than devouring stars in its path like a cosmic version of Pac-Man, this swift black hole plows into the gas ahead of it, triggering the formation of new stars along a narrow corridor. Its velocity prevents it from pausing for a celestial snack. Although never witnessed before, this remarkable occurrence was fortuitously captured by NASA’s Hubble Space Telescope.

Pieter van Dokkum of Yale University in New Haven, Connecticut, explained, “We believe we are witnessing a trail of star formation trailing the black hole, a wake behind it where the gas cools and condenses to give birth to stars. It is similar to the wake left behind a ship.” This trail must contain numerous nascent stars, evident from its luminosity, which approaches half that of the connected host galaxy.

At one extremity of this column-like formation lies an exceptionally brilliant knot of ionized oxygen, accentuating its outermost tip. Researchers theorize that the gas within the trail undergoes shock and heating due to the black hole’s rapid motion colliding with the gas, or it may result from radiation emitted by an accretion disk encircling the black hole. Van Dokkum commented, “The gas in front of it gets shocked as a result of the supersonic, high-velocity impact of the black hole moving through it. The precise mechanisms behind this phenomenon are not yet fully understood.”

Van Dokkum added, “The discovery was purely fortuitous; we stumbled upon it by chance.” Initially searching for globular star clusters in a nearby dwarf galaxy, he happened upon a faint streak while perusing the Hubble image. Initially assuming it to be an artifact caused by a cosmic ray hitting the camera detector, he soon realized that it persisted even after eliminating such interference. This enigmatic streak defied conventional explanation, standing apart from anything observed thus far.

This Hubble Space Telescope archival photo captures a curious linear feature that is so unusual it was first dismissed as an imaging artifact from Hubble’s cameras. But follow-up spectroscopic observations reveal it is a 200,000-light-year-long chain of young blue stars. A supermassive black hole lies at the tip of the bridge at lower left. The black hole was ejected from the galaxy at upper right. It compressed gas in its wake to leave a long trail of young blue stars. Nothing like this has ever been seen before in the universe. This unusual event happened when the universe was approximately half its current age. Credit: Science: NASA, ESA, Pieter van Dokkum (Yale), Image Processing: Joseph DePasquale (STScI)

Due to the peculiarity of the discovery, van Dokkum and his team conducted additional spectroscopy using the W. M. Keck Observatories in Hawaii. Describing the star trail as “astonishing, very bright, and highly unusual,” they arrived at the conclusion that it represented the aftermath of a black hole moving through a surrounding halo of gas enveloping the host galaxy.

This intergalactic phenomenon is likely the outcome of multiple collisions involving supermassive black holes. Astronomers propose that approximately 50 million years ago, two galaxies merged, resulting in the convergence of two supermassive black holes at their respective centers. These black holes began orbiting one another, forming a binary system.

Subsequently, another galaxy with its own supermassive black hole entered the scene, adhering to the adage “two’s company and three’s a crowd.” The interaction among the three black holes engendered a state of chaos and instability. One of the black holes absorbed momentum from the other two and was expelled from the host galaxy. It is plausible that the original binary system remained intact, or the newly introduced black hole may have replaced one of the members of the initial binary, expelling its former companion.

As the solitary black hole embarked on a trajectory, the binary black holes were propelled in the opposite direction. On the opposite side of the host galaxy, a feature emerges that could potentially correspond to the runaway binary black hole. Supporting this hypothesis is the absence of any indication of an active black hole remaining at the core of the galaxy. The next step involves conducting further observations using NASA’s James Webb Space Telescope and the Chandra X-ray Observatory to confirm the explanation involving the black holes.

NASA’s forthcoming Nancy Grace Roman Space Telescope, with its wide-angle view of the universe and the exceptional resolution akin to Hubble, is poised to contribute to the exploration of similar rare and improbable “star streaks” in other regions of the cosmos. Van Dokkum suggests that this endeavor might necessitate the use of machine learning algorithms proficient at identifying specific peculiar shapes amidst a vast sea of astronomical data.

The research paper was published on April 6 in The Astrophysical Journal Letters.

Reference: “A Candidate Runaway Supermassive Black Hole Identified by Shocks and Star Formation in its Wake” by Pieter van Dokkum, Imad Pasha, Maria Luisa Buzzo, Stephanie LaMassa, Zili Shen, Michael A. Keim, Roberto Abraham, Charlie Conroy, Shany Danieli, Kaustav Mitra, Daisuke Nagai, Priyamvada Natarajan, Aaron J. Romanowsky, Grant Tremblay, C. Megan Urry and Frank C. van den Bosch, 6 April 2023, The Astrophysical Journal Letters.

DOI: 10.3847/2041-8213/acba86

The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

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