Astronomers spot a rogue supermassive black hole hurtling through space leaving star formation in its wake

Supermassive black holes (SMBHs) lurk in the center of large galaxies like ours. From their commanding position in the galaxy’s heart, they feed on gas, dust, stars, and anything else that strays too close, growing more massive as time passes. But in rare circumstances, an SMBH can be forced out of its position and hurtle through space as a rogue SMBH.

In a new paper, researchers from Canada, Australia, and the U.S. present evidence of a rogue SMBH that’s tearing through space and interacting with the circumgalactic medium (CGM.) Along the way, the giant is creating shock waves and triggering star formation.

The paper is “A candidate runaway supermassive black hole identified by shocks and star formation in its wake.” The lead author is Pieter van Dokkum, Professor of Astronomy and Physics at Yale University. The paper is avaiable on the arXiv preprint server and hasn’t been peer-reviewed yet.

If you’ve never heard of a runaway SMBH, you’re not alone. SMBHs are normally locked into place at the centers of galaxies, and that’s where they stay. But scientists think that in rare circumstances SMBHs can escape their galaxies. In their paper, the authors explain how an SMBH can be cast out of its host galaxy.

It always starts when galaxies merge. That leads to the formation of a binary SMBH at the center of the merger remnant. The binary SMBH can be very long-lived, surviving for as long as one billion years before merging. If during that time, a third SMBH reaches the galactic center, then a three-body interaction can give one of the SMBHs a velocity boost, and it can be driven from the galaxy.

But despite their theoretical underpinnings, finding these rogue SMBHs is difficult. Astronomers identified one of the best candidates in 2021, about 230 million light-years away. The authors noticed peculiar movement and velocity that indicated recent disruption. But they couldn’t conclude whether they saw an ongoing galaxy merger, a binary black hole system, or a gravitational-wave recoil event.

Astronomers recognize a couple of ways that they can identify a runaway SMBH. The easiest way is if the hole is actively absorbing material as an active galactic nucleus and can be identified by its luminosity. “For such objects,” the authors write, “the presence of an SMBH is not in doubt, but it can be difficult to determine whether they are ‘naked’ black holes or the nuclei of merging galaxies.”

A rogue SMBH disconnected from its galaxy and roaming through the circumgalactic medium is a fascinating phenomenon. We’re only in the early stages of understanding SMBHs, how they’re linked with galaxy growth, and how they merge to produce gravitational waves.

To find that they can be kicked out of their host galaxies adds a layer of complexity only nature could come up with.