This amazing image, taken by the Atacama Large Millimeter Array (ALMA) in Chile, has thrown up a bit of a surprise. While the image is stunning its own right, closer examination by scientists revealed a possible dwarf dark galaxy hiding within, an intriguing type of galaxy that has eluded scientists to date.
The image is an example of what is called an Einstein Ring. This is the effect of a gravitational lens, caused when a massive object distorts the light from a more distant one, creating a “ring” as the light travels around it. This particular object is called SDP.81, or to give it its longer formal name, HATLAS J090311.6+003906.
Here, we see a galaxy 12 billion light-years away, the “red” in the image, distorted by a massive intermediary galaxy, the “blue” part of the image, 4 billion light-years away.
But in a paper published in the Astrophysical Journal, astronomer Yasher Hezaveh at Stanford University in California and his team found signs of a hidden dwarf dark galaxy in the nearby galaxy, which distorted the image of the distant galaxy. This object, highlighted as the white dot in the image above, could only be detected thanks to its gravitational influence on the more distant galaxy.
“We can find these invisible objects in the same way that you can see rain droplets on a window,” Hezaveh explained in a statement. “You know they are there because they distort the image of the background objects.”
It’s thought that this dwarf dark galaxy is hiding near the halo of its larger companion. But the fact that astronomers haven’t found more of these objects has been somewhat of a puzzle. There are thought to be thousands around our own Milky Way, but only about 40 have been found so far.
The answer may lie in the composition of the small objects. This latest observation suggests that they are made mostly of dark matter, meaning they emit little to any visible light. But their gravitational effect can still be inferred.
“Our current measurements agree with the predictions of cold dark matter,” said team member Gilbert Holder of McGill University in Montreal, Canada. “In order to increase our confidence, we will need to look at many more lenses.”