Composite image of the gravitational lens SDP.81 showing the distorted Atacama Large Millimeter/submillimeter Array (ALMA) image of the more distant galaxy (red arcs) and the Hubble optical image of the nearby lensing galaxy (blue center object). By analyzing the distortions in the ring, astronomers have determined that a dark dwarf galaxy (data indicated by white dot near left lower arc segment) is lurking nearly 4 billion light-years away.
More about this image
Subtle distortions hidden in the Atacama Large Millimeter/submillimeter Array's (ALMA) image of the gravitational lens SDP.81 are telltale signs that a dwarf dark galaxy is lurking in the halo of a much larger galaxy nearly 4 billion light-years away. This discovery paves the way for ALMA to find many more such objects and could help astronomers address important questions on the nature of dark matter.
In 2014, while studying a variety of astronomical objects to test ALMA's new, high-resolution capabilities, researchers discovered one of these images was of an Einstein ring, produced by the gravity of a massive foreground galaxy bending the light emitted by another galaxy nearly 12 billion light-years away.
Called gravitational lensing, this phenomenon was predicted by Einsteinís general theory of relativity and 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 more distant objects.
Astronomer Yashar Hezaveh of Stanford University and a team of researchers harnessed thousands of computers working in parallel for many weeks -- among them, the National Science Foundationís most powerful supercomputer Blue Waters -- to search for subtle anomalies that had a consistent and measurable counterpart in each "band" of radio data. From these combined computations, the researchers were able to piece together an unprecedented understanding of the lensing galaxyís halo, the diffuse and predominantly star-free region around the galaxy, and discovered a distinctive clump less than one-thousandth the mass of the Milky Way.
Because of its relationship to the larger galaxy, estimated mass, and lack of an optical counterpart, the astronomers believe this gravitational anomaly may be caused by an extremely faint, dark-matter dominated satellite of the lensing galaxy. According to theoretical predictions, most galaxies should be brimming with similar dwarf galaxies and other companion objects. Detecting them, however, has proven challenging. Even around our own Milky Way, astronomers can identify only 40 or so of the thousands of satellite objects that are predicted to be present.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities Inc.
To learn more about this research, see the NSF News From the Field story Dwarf dark galaxy hidden in ALMA gravitational lens image. (Date image taken: unknown; date originally posted to NSF Multimedia Gallery: Jan. 2, 2018)