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Astronomers detect rogue supermassive black hole hurtling through space leaving star formation in its wake
February 16, 2023
Astronomers detect rogue supermassive black hole hurtling through space leaving star formation in its wake
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Supermassive black holes (SMBHs) lurk at the center of large galaxies like our own. From their commanding position at the heart of the galaxy, they feed on gas, dust, stars, and anything else that gets too close, growing more massive as time goes on. But in rare circumstances, an SMBH can be forced out of position and launched into space like a rogue SMBH.

In a new paper, researchers from Canada, Australia and the US present evidence of a rogue SMBH traversing space and interacting with the circumgalactic medium (CGM). Along the way, the giant is creating shock waves and causing the formation of stars.

The paper is “A runaway supermassive black hole candidate identified by collisions and star formation in its path. The lead author is Pieter van Dokkum, a professor of astronomy and physics at Yale University. The document has not yet been peer reviewed.

If you’ve never heard of a runaway SMBH, you’re not alone. SMBHs are normally locked in the center of galaxies, and that’s where they stay. But scientists believe that, in rare circumstances, SMBHs can escape their galaxies. In their article, the authors explain how an SMBH can be ejected from 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 up to a 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 speed boost and it can be ejected from the galaxy.

But despite its theoretical underpinnings, finding these rogue SMBHs is difficult. The astronomers identified one of the best candidates in 2021, about 230 million light-years away. The authors noted a peculiar movement and speed that indicated a recent interruption. But they couldn’t conclude whether they saw an ongoing galaxy merger, a black hole binary 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 like 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 may be difficult to determine whether they are ‘naked’ black holes or merging galaxy nuclei.”

Another way is by the stellar mass that drags the rogue hole. When an SMBH is ejected, its massive gravitational power pulls some stars with it. But without the luminosity of an AGN, the dimness makes it difficult to identify the hole and its stellar companions from a great distance.

A third way astronomers might recognize a possible rogue SMBH is by the effect it has on diffuse gas in the circumgalactic medium (CGM) as it passes through it.

“The interaction of a runaway supermassive black hole with the CGM may lead to the formation of a trail of impacted gas and young stars behind it,” the authors write. In their article, they report the serendipitous discovery of a linear feature in Hubble’s Advanced Camera for Surveys images that could be one of these contrails.

When an SMBH travels through ionized hydrogen in the CGM, it produces a shock front with a long wake behind it. In the wake, clouds of shocked gas can cool and form stars that look like bumps in the road. The researchers analyzed three of the knots in the linear feature and measured their age and metallicity.

Theory and models show that the youngest stars formed in the wake should be less than 30 million years old. When the researchers measured their properties, they found that all three knots are within the age range established by the models. They are also within the range of metallicities and dust content.

If the host galaxy ejected an SMBH, the galaxy should show signs of disruption. SMBHs are extraordinarily massive, and that amount of mass cannot move through a galaxy without giving it shape. The team examined the galaxy that spawned the rogue SMBH and found that its morphology was altered.

However, there is another possible explanation for the linear characteristic. It could be a black hole jet instead of a rogue SMBH. Under the right conditions, black hole jets can also impact gas in the CGM and lead to star formation. “There are two well-studied nearby examples of jets triggering star formation,” the authors state. One of them is called the Minkowski Object.

The authors acknowledge that the black hole jet explanation is a possibility, but say it has too many problems. Visible emission lines are not a property of black hole jets, and there is no evidence of nuclear activity. There is also a problem with morphology. “A more serious problem is that the morphology of the feature does not match simulations or observations of jet-induced star formation,” the authors explain.

They decide on a runaway SMBH as the best explanation for the data and observations.

This research schematic shows how a rogue black hole is born. In 1 and 2, a merger leads to the formation of a long-lived binary SMBH. At 3, a third galaxy enters and its SMBH plunges into the center of the new remnant. This creates a three-in-4 body interaction. The less massive SMBH breaks away from the other two and takes a speed hit in the opposite direction. 5 shows that if there is a powerful enough speed kick, then it is possible that all three SMBH can be kicked out. 6 is a chart from Illustris TNG showing that the circumgalactic medium can have highly asymmetric flows, and the SMBH in A travels through such a relatively dense and cold CGM region. Image credit: van Dokkum et al. 2023