If you see a similar streak in one of your photos, you would probably take some time to clean the lens. But the streak, in this case, was in an image taken by the Hubble Space Telescope, which is unaffected by the schmutz everyday life leaves behind on Earth-bound hardware. So a team of researchers decided to find out what the long, thin smear might represent.
They’re still not sure, but the best explanation seems to be the wake left by a supermassive black hole that erupted from the galaxy it used to be home to. The liberation was likely due to two extra supermassive black holes, all brought together by a galaxy merger. If true, this is the first instance of this behavior we’ve ever seen.
What is that?
In the days of film cameras, when it was sometimes possible to pass months or even years between taking a picture and having it developed, it was not uncommon to pick up your newly developed snaps and ask yourself what you had made a picture of. You can almost hear echoes of those days in astronomers’ description of seeing the smear across one of Hubble’s images: “an almost straight, thin streak was immediately visible upon a visual assessment of the data quality.”
Closer inspection revealed that the stripe extended toward a somewhat odd-looking galaxy. “Because we hadn’t encountered anything like this before in our own images or in the literature, we decided to include the feature in the observation plan for a planned Keck [telescope] walk.”
The streak (in the center of both images) appears to originate in a galaxy at the upper right at two different wavelengths.
van Dokkum, et. already.
Based on the redshift of the object’s light, both the object and the galaxy are approximately the same distance from Earth, suggesting that the two are related. The researchers estimate that the streak is about 200,000 light-years long. While the supermassive black holes at the center of galaxies can emit jets of material for so long (and even longer), those jets tend to spread out as they get farther from the galaxy. In this case, the stripe remained thin along its entire length.
A look at the emission from the stars present in the streak suggests that the stars generally get younger as you move further away from the galaxy. Putting it all together, it seems that the streak began to form about 40 million years ago, and since then the tip has been gradually moving away from the galaxy at about 1,000 miles per second.
Old theories
A possible explanation for this movement is that the galaxy has ejected a supermassive black hole. This is unavoidable because of two observations: almost all galaxies appear to have a supermassive black hole at their core, and most galaxies were created by multiple mergers. As a result, the supermassive black holes of the pre-fusion galaxies will eventually collide. There are two ways this can lead to an ejection. One is that if two of these supermassive black holes merge with the production of gravitational energy uneven, that could give a directional kick to the post-merger product.
An alternate pathway to ejection occurs if one or more galaxy mergers are relatively fast (in astronomical terms) mergers, it is possible that not all of their central black holes have merged yet. In these cases, you may be able to get three or more of these giants orbiting each other, allowing gravitational interactions to eject one.
