Astronomers Discovered a Dwarf Dark Matter Galaxy Hiding in This Image

April 19, 2016 | Joanne Kennell

Composite image of the gravitational lens SDP.81 showing the distorted ALMA image of the more distant galaxy (red arcs) and the Hubble optical image of the nearby lensing galaxy (blue center object).
Photo credit: Y. Hezaveh, Stanford Univ.; ALMA (NRAO/ESO/NAOJ); NASA/ESA Hubble Space Telescope

Galaxies like our Milky Way contain stars, but dwarf dark matter galaxies consist of something else entirely.

Galaxies like our Milky Way are massive collections of stars. In fact, a single galaxy can contain hundreds of billions of them. But would you believe me if I told you there is a type of galaxy that has no stars at all? These galaxies are known as dwarf dark galaxies, and instead of consisting of stars, they contain mostly dark matter.

Current dark matter theory suggests that there should be thousands of these dwarf dark galaxies in the halo — predominantly star-free region around a galaxy — around regular galaxies, but discovering them has not been an easy task.

Now, in a paper accepted for publication in the Astrophysical Journal, astronomer Yashar Hezaveh at Stanford University in California, and his colleagues, explained how the detailed analysis of a widely publicized image taken in 2014 using the Atacama Large Millimeter Array (ALMA) telescope, has uncovered signs of a hidden dwarf dark galaxy in the halo of a more nearby galaxy.

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The image shows an Einstein ring that was produced by the gravity of a massive foreground galaxy bending the light emitted by another galaxy nearly 12 billion light-years away. This phenomenon, called gravitational lensing, was predicted by Einstein’s general theory of relativity, and it is a powerful tool for studying galaxies that are otherwise too distant to observe.

Gravitational lensing can shed light on the properties of the nearby galaxy because of the way its gravity distorts and focuses light from more distant objects.

"We can find these invisible objects in the same way that you can see rain droplets on a window. You know they are there because they distort the image of the background objects," explained Hezaveh in a press release. In the case of a raindrop, the image distortions are caused by refraction. However, similar distortions are generated by the gravitational influence of dark matter.

Current theories suggest that dark matter, which makes up about 27 percent of the mass of the universe, is made of particles that don't interact with visible light or other forms of electromagnetic radiation. However, dark matter does have appreciable mass, so it can be identified by its gravitational influence on other objects.

According to theoretical predictions, most galaxies should have a ton of these smaller galaxies within their halos, but detecting them has been challenging. In fact, around the Milky Way, astronomers have identified just 40 or so of the thousands of objects that are predicted to be there.

"This discrepancy between observed satellites and predicted abundances has been a major problem in cosmology for nearly two decades, even called a 'crisis' by some researchers," said Neal Dalal of the University of Illinois, a member of the team, in the release.

"If these dwarf objects are dominated by dark matter, this could explain the discrepancy while offering new insights into the true nature of dark matter," Dalal continued.

However, in order to prove that these galaxies are in fact dominated by dark matter, astronomers will have to observe many more Einstein rings. Luckily, thanks to the power of ALMA, that task has become substantially easier.

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