In an astonishing feat of gravitational research, astronomers have discovered a mysterious, dense blob of invisible matter buried in a galaxy whose light took 7.3 billion years to reach us.
Exactly what this drop could be is currently an open question, but it is absolutely tiny compared to the distance at which it was detected – about a million times the mass of the Sun. It is the smallest object discovered by gravity at large cosmic distances, by a factor of about 100.
“This is the lowest known mass object, by two orders of magnitude, that can be detected at a cosmological distance by its gravitational effect,” explains a team led by astrophysicist Devon Powell of the Max Planck Institute for Astrophysics in Germany.
“This work demonstrates the observational feasibility of using gravitational imaging to probe the million-mass solar regime well beyond our local Universe.”
Related: Mysterious dark matter mapped in space like never before
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Based on our observations of the Universe, there is something that emits no light and only interacts with the rest of the Universe through gravity.
We call this something dark matter, and there are several possible explanations for what it could be. The consistency of the material – whether smooth or lumpy – can help scientists narrow it down. However, because dark matter does not emit any light, it is difficult to map its distribution.
This brings us to gravity. Anything that has mass in the Universe causes spacetime to curvature around it – the greater the mass, the greater the curvature of spacetime. Imagine putting, say, a bowling ball on a trampoline. If you roll a ball on the taut trampoline mat, it will follow the curved path around the bowling ball.
Now imagine that the bowling ball is a galaxy and the ball is a photon. A collection of photons from a distant galaxy traveling through the gravity-warped spacetime of a closer galaxy (the bowling ball) will come to us stretched, warped and enlarged. This is what we call gravitational lensing.
These lenses are a brilliant tool for studying the distant Universe, because they magnify deep space in ways that technology cannot. But astronomers can also use this stretched and distorted distant light to map the distribution of matter in the foreground lens.
That’s what Powell and his colleagues set out to do, using a large array of telescopes, including the Green Bank Telescope, the Very Long Baseline Array, and the European Very Long Baseline Interferometric Array, to focus on a well-known gravitational lensing system called JVAS B1938+666.
This system consists of a foreground galaxy with a light travel time of about 7.3 billion years and a more distant galaxy with a light travel time of about 10.5 billion years whose light has been stretched and quadrupled by the foreground galaxy.
One of the images of the lensed galaxy is a bright, spreading arc of light; in this stained arch, the researchers found a sort of pinched dimple. This pinch, the researchers found, could not have been created by the lensing galaxy alone. Instead, the culprit must be a mass cluster, a determination made with a whopping 26 sigma confidence level.
“From the first high-resolution image, we immediately observed a narrowing of the gravitational arc, which is the telltale sign that we were on to something,” says astronomer John McKean of the University of Groningen in the Netherlands.
“Only another small cluster of mass between us and the distant radio galaxy could cause this.”
The mass does not emit any light – not in optical, radio or infrared wavelengths. It’s either completely dark or way too dark to see. That means it could be several things. The leading candidates are a dark matter cluster or dwarf galaxy that emits too little light for us to detect.
Either option is plausible at this time and further research is needed to determine the identity of the culprit.
“Given the sensitivity of our data, we expected to find at least one dark object, so our discovery is consistent with the so-called ‘cold dark matter theory’ on which much of our understanding of galaxy formation is based,” Powell said.
“Having found one, the question now is whether we can find more and whether their numbers will still agree with the models.”
The results were detailed in complementary articles published in Natural astronomy and the Monthly Notices of the Royal Astronomical Society.
