Dark matter-rich dwarf galaxy found hidden in halo of lensing galaxy

Evidence of a dwarf dark galaxy hidden in the halo of another larger, gravitationally-lensed galaxy may be the first of many such objects to be discovered by astronomers, and could help solve the mystery of dark matter, National Radio Astronomy Observatory officials have revealed.

As the NRAO explained in a statement Thursday, they discovered subtle distortions in an image of SDP.81, an active star-forming galaxy located 12 billion light-years away that is being lensed by a massive foreground galaxy located a relatively close four billion light-years from Earth.

The image, which was taken by the Atacama Large Millimeter/submillimeter Array (ALMA) as part of ALMA’s Long Baseline Campaign in 2014, was of a phenomenon called an Einstein ring. However, as Stanford University researcher Yashar Hezaveh and his colleagues explain in a new study, it also contained hidden evidence of a dwarf dark galaxy in the nearby galaxy’s halo.

“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,” the Stanford astronomer said. His team’s research is currently available online and has also been accepted for publication in an upcoming edition of The Astrophysical Journal.

Dark matter content may have made dwarf galaxies hard to find

While in the case of a rain drop, the image distortions are caused by refraction, Hezaveh and his colleagues explained that the distortions in the image are generated by the gravitational influence of dark matter – mysterious particles that do not interact with visible light and which are thought to comprise as much as 80 percent of the total mass of the universe.

Although dark matter has yet to be identified, it does have appreciable mass and can be identified by the gravitational influence it exerts on other objects, the study authors explained. So they used thousands of computers to hunt for subtle anomalies with consistent and measurable counterparts in each band of radio data, which they used to analyze the halo of the lensing galaxy.

They discovered a lump, roughly one one-thousanth the mass of the Milky Way, in this primarily star-free region surrounding the galaxy. Based on its estimated mass, its relationship to the larger galaxy and the lack of an optical counterpart, Hezaveh’s team believes that this anomaly may be an extremely faint, dark-matter rich satellite of the lensing galaxy. Theoretically, the majority of galaxies should contain such dwarf galaxies, but detecting them has been difficult.

“This discrepancy between observed satellites and predicted abundances has been a major problem in cosmology for nearly two decades,” said co-author Neal Dalal, a researcher at the University of Illinois. “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.”

“This is an amazing demonstration of the power of ALMA,” Hezaveh added. “We are now confident that ALMA can efficiently discover these dwarf galaxies. Our next step is to look for more of them and to have a census of their abundance to figure out if there is any possibility of a warm temperature for dark matter particles.”

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Image Credit: Y. Hezaveh, Stanford Univ.; ALMA (NRAO/ESO/NAOJ); NASA/ESA Hubble Space Telescope

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