Have you ever been driving on a freeway behind a large moving van and the back doors suddenly open and its contents start flying out in all directions? Now imagine that you are in a starship and you’re flying behind a speeding supermassive black hole when it suddenly begins spewing out stars from the galaxy it just moved out of. That is the scenario astronomers using the Hubble Telescope discovered recently while looking for something else – isn’t that the way it always works? They found a supermassive black hole estimated to be 20 million times the mass of the sun speeding away from its home galaxy at 3.5 million mph (5.6 million km/h), or about 4,500 times the speed of sound. You wouldn’t need a sonic boom to find it – this black hole was trailed by a tail more than 200,000 light-years long filled with stars it may have dragged out of its host galaxy. What happened and could this same scenario occur in the Milky Way?
“The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to the formation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of an extremely narrow linear feature in HST/ACS images that may be an example of such a wake.”
“Serendipitous” sounds so much better than “accidentally” so lead author Pieter van Dokkum, professor of physics and astronomy at Yale University, and his team used it to describe how they found “A candidate runaway supermassive black hole identified by shocks and star formation in its wake” – the title of the paper in the preprint journal arXiv and accepted for publication in The Astrophysical Journal Letters. They were using the Hubble space telescope to study the dwarf galaxy RCP 28 located about 7.5 billion light-years from Earth. Like all astronomers would be, they were bothered by what appeared to be a smudge in the image. However, a closer look with the Keck telescope in Hawaii showed this was no smudge – it appeared to be a trail of gas and newly formed stars. As astronomers and hikers know, following a trail eventually takes you to its beginning or trailhead. In this case, the head was not a road or a shelter but a supermassive black hole speeding away from something and leaving this trail or smudge or streak behind it. (Images can be seen here.)
Supermassive black holes are normally anchored in the center of a galaxy. What is this monster doing outside of one?
“From a detailed analysis of the feature, we inferred that we are seeing a very massive black hole that was ejected from the galaxy, leaving a trail of gas and newly formed stars in its wake."
While some media reports referred to this supermassive blank hole as a “runaway,” it soundsmore like it was kicked out. The astronomers decided to work backwards from the point they observed it and its trail to its source as a way to figure out what happened. The mysteries only worsened as they noticed that what they thought were normal jets of gas spewing out of it and stars that the black hole had dragged away from its galaxy were both something else. As Live Science explains, supermassive black holes eject astrophysical jets of material at high speeds, and these jets appear to be streaks of light – this is one of the ways astronomers can ‘see’ a black hole. Also, typical jets weaken as they move farther away from the black hole’s gravity. However, the closer look at this black hole’s trail showed this was not the kind of astrophysical jet stream they were looking for. For one thing, these jets of gas appeared to get stronger as they moved away from the black hole. Also, astrophysical jets are normally spewed out in a fan shape – these were in a straight line.
Then there are the stars which are also trailing behind this black hole. Because it was ejected, probably violently, it was expected that these stars were also either ejected or pulled out of the galaxy by the speeding black hole. That theory got a big black hole blasted through it when the astronomers determined that these were newly formed stara that were being created as the supermassive black hole streaked along in space. Could this be some kind of weird way new galaxies form? That is a strong possibility. But first, the astronomers have to figure out how it got violently kicked out of its galaxy in the first place. Pieter van Dokkum has a theory.
"The most likely scenario that explains everything we've seen is a slingshot, caused by a three-body interaction. When three similar-mass bodies gravitationally interact, the interaction does not lead to a stable configuration but usually to the formation of a binary and the ejection of the third body."
What van Dokkum is suggesting is that it wasn’t one galaxy that did the ejecting but a collision between a rare galaxy with a binary supermassive black hole and a second galaxy with just one. As they merged, there was a galactic version of ‘Two’s company … three’s a crowd’ and the new binary flung the third black hole into space, trailing gas behind it – gas that was forming new stars that might one day form a new galaxy when this black hole stopped and found an open area to anchor it in. As usual in astronomy, if you see one, there are probably more, and van Dokkum agrees.
"Ejected supermassive black holes had been predicted for 50 years but none have been unambiguously seen. Most theorists think that there should be many out there."
While all of the evidence points to an ejected supermassive black hole, the team is still looking for more to build a better case. Also, they wonder if the only cause of an ejected black hole is a three black hole collision of if they can be kicked out of their galaxy in some other violent fashion. The good news is, there don’t seem to be any galaxies with binary supermassive black holes speeding on a collision course with the Milky Way.