Researchers have uncovered the existence of a dwarf "dark galaxy" lurking nearly 4 billion light-years away from Earth. The discovery was made when a team of researchers, including astronomers at the University of Illinois, using the Blue Waters supercomputer at the National Center for Supercomputing Applications (NCSA), noticed subtle distortions in the image of gravitational lens SDP.81. The discovery paves the way to spot many more such objects, which could help astronomers address important questions on the true nature of dark matter.

The discovery came as part of a campaign to test new high-resolution capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA), an observatory in northern Chile run by an international partnership of science organizations. One of these experimental images was that of an Einstein ring, which was produced by 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 offers a powerful tool for studying galaxies that are otherwise too distant to observe. It also sheds light on the properties of the nearby lensing galaxy because of the way its gravity distorts and focuses light from the more distant galaxy.

According to a paper to be published in the Astrophysical Journal, detailed analysis of the new ALMA image uncovered signs of a hidden dwarf galaxy in the halo of the more nearby galaxy.

Current theories suggest that dark matter, which makes up 80 percent of the mass of the universe, is made of as-yet-unidentified particles that don't interact with visible light or other forms of electromagnetic radiation. Dark matter does, however, have appreciable mass, so it can be identified by its gravitational influence.

The Illinois Connection
For their analysis, the researchers harnessed thousands of computers working in parallel for many weeks, including NCSA's Blue Waters supercomputer, one of the most powerful supercomputers in the world, to search for subtle anomalies that had a consistent and measurable counterpart in each "band" of radio data.

From that analysis, the researchers were able to piece together an unprecedented understanding of the lensing galaxy's halo, the diffuse and predominantly star-free region around the galaxy, and discovered a distinctive clump less than one-thousandth the mass of our galaxy.

Joaquin Vieira, a professor of astronomy and co-author on the paper, said researchers at Illinois had an "enormous" footprint on the project as they brought together local observers, theorists, and supercomputing experts to work on the discovery. Other co-authors at Illinois included Neal Dalal, professor of astronomy; Athol Kemball, professor of astronomy and NCSA expert in interferometry and big data; and Di Wen, a graduate student in astronomy who ran the analysis code.

Researchers at Stanford University, the University of Arizona, McGill University, and other institutions were also involved with the research.

"We used the largest telescope on Earth, along with an effect of space-time predicted by Einstein (gravitational lensing), along with the largest National Science Foundation-funded supercomputer (Blue Waters), to take the most sensitive picture of dark matter ever," Vieira said.

"This technique is going to become more important over the next few years, and we hope that with larger samples of similar objects discovered in the next few years, we'll learn a lot more about the properties dark matter, like the mass of the dark matter particle. This weighs in on one of the most profound outstanding questions in cosmology from the last 60 years: what is the nature of dark matter?"

According to theoretical predictions, most galaxies should be brimming with similar dwarf galaxies and other companion objects. Detecting them, however, has proven challenging. Even in our own Milky Way, astronomers can identify only 40 or so of the thousands of satellite dwarfs that are predicted to be present.

"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. "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," he added.

Finding More Dwarfs
Computer models of the evolution of the universe indicate that by measuring the "clumpiness" of dark matter, it's possible to measure its temperature. So by counting the number of small dark matter clumps around distant galaxies, astronomers can infer the temperature of dark matter, which has an important bearing on the smoothness of our universe.

"If these halo objects are simply not there," Daniel Marrone of the University of Arizona, one of the co-authors notes, "then our current dark matter model cannot be correct and we will have to modify what we think we understand about dark matter particles."

This study suggests, however, that the majority of dwarf galaxies may simply not be seen because they're mainly composed of invisible dark matter and emit little if any light. "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."

"This is an amazing demonstration of the power of ALMA," said Hezaveh. "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."

Research paper: "Detection of Lensing Substructure Using ALMA Observations of the Dusty Galaxy SDP.81," Yashar Hezaveh et al., 2016, Astrophysical Journal