Dark matter … spacetime warp … dwarf galaxies … Einstein ring. Those sound like plot points for the next Star Trek movie but they’re actually from a new study about the discovery of a hidden dwarf galaxy that proves once again the genius of Albert Einstein.
This story begins in 2014 when astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile during the Long Baseline Campaign (a test from September to December 2014 to test ALMA’s power) observed SDP.81, a gravitational lens formed by a distant galaxy whose light was warped by the gravitational field of a galaxy in front of it – a classic Einstein ring from the theory of general relativity, which predicts spacetime can become bent by a powerful gravitational field.
In this case, the gravitational lensing indicated the forefront galaxy was exactly positioned to create what looked like a perfect Einstein ring, but closer examination showed distortions. Those distortions indicated that the galaxy in front – 12 billion light-years away – was massive, while the galaxy it blocked was no ordinary galaxy.
According to a paper in the Astrophysical Journal authored by astronomer Yashar Hezaveh at Stanford University and others, the hidden galaxy is actually a small, low-mass, dim dwarf galaxy made of dark matter. This is a big deal because astronomers have predicted that the galaxies should be full of these dwarf galaxies – the Milky Way was expected to have thousands – yet very few have been found – only 40 in the Milky Way so far. Team member Neal Dalal of the University of Illinois is excited about this new discovery.
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. 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.
So the dwarf galaxies haven’t been found because they’re composed of invisible dark matter and create very little light. Now astronomers have a means to search for more gravitational lenses that may be hiding dwarf galaxies loaded with dark matter, says Yashar Hezaveh.
This is an amazing demonstration of the power of ALMA. 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.
Einstein was right again! Our long missing dwarf galaxy crisis is over!