According to the widely-accepted ‘cold dark matter’ theory, all galaxies are embedded within clouds of dark matter. Dark matter itself consists of slow-moving particles that come together to form structures ranging from hundreds of thousands of times the mass of our Milky Way Galaxy to small-scale clumps. The new observations by the NASA/ESA Hubble Space Telescope yield insights into the nature of dark matter and how it behaves.
Each of these Hubble images reveals four distorted images of a background quasar and its host galaxy surrounding the central core of a foreground massive galaxy. Image credit: NASA / ESA / A. Nierenberg, NASA’s Jet Propulsion Laboratory / T. Treu, University of California, Los Angeles.
While dark matter concentrations have been detected around large- and medium-sized galaxies, much smaller clumps of dark matter have not been found until now.
In the absence of observational evidence for such small-scale clumps, some astrophysicists have developed alternative theories, including ‘warm dark matter.’ This idea suggests that dark matter particles are fast moving, zipping along too quickly to merge and form smaller concentrations.
The new Hubble observations do not support this scenario, finding that dark matter is ‘colder’ than it would have to be in the warm dark matter alternative theory.
“Dark matter is colder than we knew at smaller scales,” said Dr. Anna Nierenberg, an astronomer at NASA’s Jet Propulsion Laboratory.
“Astronomers have carried out other observational tests of dark matter theories before, but ours provides the strongest evidence yet for the presence of small clumps of cold dark matter.”
“By combining the latest theoretical predictions, statistical tools and new Hubble observations, we now have a much more robust result than was previously possible.”
Hunting for dark matter concentrations devoid of stars has proved challenging. Dr. Nierenberg and her colleagues, however, used a technique in which they did not need to look for the gravitational influence of stars as tracers of dark matter.
They targeted eight powerful quasars, which are located roughly 10 billion light-years from Earth.
They measured how the light emitted by oxygen and neon gas orbiting each of the quasars’ black holes is warped by the gravity of a massive foreground galaxy, which is acting as a magnifying lens. The lensing galaxies are located at a distance of about 2 billion light-years away from Earth.
Using this method, they uncovered clumps of dark matter along the Hubble’s line of sight to the quasars, as well as in and around the intervening lensing galaxies.
“Imagine that each one of these eight galaxies is a giant magnifying glass,” said Dr. Daniel Gilman, an astronomer at the University of California, Los Angeles.
“Small dark matter clumps act as small cracks on the magnifying glass, altering the brightness and position of the four quasar images compared to what you would expect to see if the glass were smooth.”
The dark matter concentrations detected by the telescope are 1/10,000th to 1/100,000th times the mass of the Milky Way’s dark matter halo.
Many of these tiny groupings most likely do not contain even small galaxies, and therefore would have been impossible to detect by the traditional method of looking for embedded stars.
The eight quasars and galaxies were aligned so precisely that the warping effect, called gravitational lensing, produced four distorted images of each quasar.
Such quadruple images of quasars are rare because of the nearly exact alignment needed between the foreground galaxy and background quasar.
However, the astronomers needed the multiple images to conduct a more detailed analysis.
The presence of the dark matter clumps alters the apparent brightness and position of each distorted quasar image.
The researchers compared these measurements with predictions of how the quasar images would look without the influence of the dark matter.
They used the measurements to calculate the masses of the tiny dark matter concentrations.
“We made a very compelling observational test for the cold dark matter model and it passes with flying colors,” said Dr. Tommaso Treu, an astronomer at the University of California, Los Angeles.
The scientists presented the results this week at the 235th Meeting of the American Astronomical Society in Honolulu, Hawai’i.
_____
J.R. Lu et al. 2020. KAPA: A new Keck laser-guide star AO system that increases image quality and sky coverage. 235th AAS Meeting, abstract # 118.03
D. Gilman. 2020. Constraints on the nature of dark matter with quadruple-image strong gravitational lenses. 235th AAS Meeting, abstract # 133.05
This article is based on text provided by the National Aeronautics and Space Administration.
