Black holes are kind of scary. Think about an invisible pitfall that swallows everything that gets close. Even light, the fastest thing in the universe, can’t escape them! From a scientist’s point of view, however, black holes are kind of cool. Their extreme nature makes them a very interesting thing to study, full of weird antics and head-scratching mysteries. Sitting at a nice and safe distance here on Earth, astronomers have been observing all sorts of weird events related to them, doing their best to understand and explain them.
Even though some people have been studying these unique objects for years, the universe is a vast place, and still sometimes throws a curveball at us that leaves even experts wondering. One such event was observed in September 2022, when researchers from Yale University were inspecting their recent observations from the Hubble Space Telescope. They saw an unusual feature, that looked like an extremely long and thin straight line, next to a galaxy over 10 billion light years away.
The Linear Feature. Image Credit: Van Dokkum et al., 2023
At first the researchers thought they had missed a stray cosmic ray while processing their observations, which then lit up just a few pixels of their camera. Thankfully, the telescope can observe multiple different kinds of light through its different filters, and a single cosmic ray should only be visible at one filter. Weirdly enough, the line feature could be seen in all observations regardless of filter, so it could not have been due to a stray ray. In reality, it was something a lot more exciting.
Looking deeper into it, the thin line pointed straight at the centre of a nearby dwarf galaxy. Dwarf galaxies, while not as big or bright as our own Milky Way, are still huge structures, and the weird line was almost half as bright as its neighbour. By examining the light properties observed from the line and the galaxy, the astronomers found that the two objects contained some similar chemical elements and were also at the same distance from the Earth. This was important, because the line appeared to be some 200 thousand light years long, even longer than the Milky Way galaxy, and much longer than its dwarf companion. Whatever this odd line was, it was absolutely massive! Not having a clear idea what to make of it, the astronomer decided to look at this odd pair again with their next set of observations.
After taking a closer look, they found that the line was not the only unusual thing about this. The galaxy it pointed at was examined carefully and found to have some interesting qualities. It was small and compact, irregular in shape, and was forming stars at a much higher rate than expected for a galaxy of its kind, almost 10 times higher! These results suggested that this galaxy had changed drastically not too long ago. About 100 million years in its past (which is not that long for a galaxy!), the galaxy had merged with another dwarf galaxy in a spiralling celestial dance. The result of two galaxies merging is a new, brighter galaxy, with higher rates of star formation and the total stars of its two previous parts. But a merge like this can have other, unexpected effects.
A galaxy is a huge structure containing billions of stars held together by gravity. The heart of a galaxy is its brightest part, thanks to many many stars gathering close together, lighting it up brilliantly. Despite its warm light though, a galaxy’s heart contains another, darker part. A giant black hole, weighing millions of times the mass of the Sun, lurks in the heart of every galaxy. That’s over a billion trillion trillion tons! When two galaxies collide and merge, the black holes in their hearts also start to interact. Usually the two combine into another, bigger black hole, but if a third galaxy comes along before the merge is complete, we get a third giant black hole to interact with the merging pair. The third black hole can “kick” the smaller of the pair and send it flying out and away from the galaxy at blazing speed. This is an extremely rare event to observe, so we are still not sure of the exact sequence of interactions that occur, or how it would look from so far away!
The process of a merging pair of galaxies encountering a third one. The black holes at the centres interact intricately, and one can even get expelled! Image Credit: Van Dokkum et al., 2023
Could this really be what caused the bizarre line in the observations? The astronomers considered many different possible explanations. At first they thought it could be a super-thin galaxy seen from the side, but that does not fit with the symmetry of the feature, its high brightness and its intense star formation rates. All the most viable explanations required the presence of a super-massive black hole, so next they thought the observed line could be a massive jet coming from an active galactic centre. That would be easy to verify, because those always produce very high amounts of X-rays. The observed feature showed almost no X-rays, so that explanation was also ruled out. It could be that a jet had been there in the past coming from the “host” dwarf galaxy which had shocked the region into high star formation and had then turned off. In that case, we would see the shocked region getting wider as it moved away from the galaxy, opening up like a cone. The observed feature was actually getting narrower as it moved away from the galaxy, like it was created by something leaving the galaxy instead of by the galaxy itself. The astronomers were left with only one explanation: a giant black hole had been ejected from the galaxy and was now travelling through the universe, shocking interstellar space in its path and causing stars to form!
Artist’s impression of the runaway black hole moving away from its former host galaxy. Image Credit: NASA, ESA, Leah Hustak (STScI)
This cosmic bullet is a truly rare and mind-boggling event. A hungry black hole, 40 times larger than the Sun and weighing 10 million times more, is hurling through space with a speed calculated at 16,000 kilometers per second, 4,000 times faster than a rocket ship. The leading researcher of this finding, experienced extragalactic astronomer Pieter van Dokkum, called this a “serendipitous discovery” and a “remarkable feature”. Only a few of these events have been found so far by all of mankind, but that could soon change. This is only one of many wondrous astronomical phenomena that we can observe as of right now. With the progress of modern science and technology, our telescopes and our instruments are getting bigger, better and stronger every year. In the coming years and decades we are sure to discover all sorts of fantastical events that will undoubtedly challenge our understanding and leave us searching for answers, with curiosity as our number one tool.
Sources:
- A Candidate Runaway Supermassive Black Hole Identifed by Shocks and Star Formation in its Wake, van Dokkum, P. et al 2023, The Astrophysical Journal Letters, Volume 946, Number 2
- NASA, ESA, Leah Hustak (STScI)
Written by Kyriakos Trakakis
