“Dark matter holds the key to understanding the universe,” says astrophysicist Paul Davies. ”After the big bang that created the universe 13.7 billion years ago, matter was spread smoothly through space, but aided by the gravitating power of the dark component, ordinary matter was drawn into clumps, which later evolved into galaxies that spawned stars, planets and, in one case at least that we know of, life.”
Davies observation, which is shared by most scientists, may have been turned on its head by the discovery in November, 2019, by astronomers from the National Astronomical Observatories of the Chinese Academy of Science (NAOC), Peking University and Tsinghua University who, searching the sky for yet-undiscovered galaxies which seem to be lacking the usual component of dark matter, identified 19 galaxies which might violate the most fundamental theory of how the universe first formed.
Confounds Standard Galaxy Formation Model
“This result is very hard to explain using the standard galaxy formation model,” said lead author Qi Guo of the Chinese Academy of Science in a press release about the special population of dwarf galaxies that could mainly consist baryons within radii of up to tens of thousands of light-years. This contrasts with the normal expectation that such regions should instead be dominated by dark matter.
In standard cosmology, the Universe is dominated by cold dark matter and dark energy, while baryons only occupy 4.6% by mass. Galaxies form and evolve in systems dominated by dark matter. In high-mass systems, the baryonic fraction may reach the universal value, i.e., 4.6%. In low-mass systems, the baryonic fraction may be much lower due to their shallow gravitational potential.
The satellite dwarf galaxies in our Local Group are found to be dominated by dark matter down to radii of a few thousand light-years. However, statistical studies of the dynamics of dwarf galaxies beyond the Local Group previously had been hampered by the extreme faintness of such systems. but Multi-wavelength data have recently made such studies possible, however.
By taking advantage of the release of 40% of the data from the Arecibo Legacy Fast (ALFA) catalogue and the Seventh Data Release of the Sloan Digital Sky Survey, a research group led by Prof. Guo Qi from NAOC has found 19 dwarf galaxies that are dominated by baryons at radii far beyond their half-optical radii ( typically a few thousand light-years). Normally, the dark matter-to-baryon mass ratio reaches 10-1000 for “typical” dwarf galaxies. Notably, most of these baryon-dominated dwarf galaxies are isolated galaxies, free from the influence of nearby bright galaxies and high-density environments.
“This result is very hard to explain using the standard galaxy formation model in the context of concordance cosmology, and thus encourages people to revisit the nature of dark matter,” said Prof. GUO.
Instead of the standard cold dark matter model, a warm dark matter model or fuzzy dark matter model might be more in line with the formation of this particular population of dwarf galaxies. Alternatively, some extreme astrophysical processes may also be responsible.
Further observations are required to understand the formation of these particular baryon-dominated dwarf galaxies.
“We thought that every galaxy had dark matter and that dark matter is how a galaxy begins,” said Pieter van Dokkum, Yale’s Sol Goldman Family Professor of Astronomy and lead author of the 2018 study in the journal Nature suggesting that the galaxy shows for the first time that dark matter is not always associated with traditional matter on a galactic scale, ruling out several current theories that dark matter is not a substance but merely a manifestation of the laws of gravity on cosmic scales.
“This invisible, mysterious substance is the most dominant aspect of any galaxy,” van Dokkum said. “So finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real. It has its own separate existence apart from other components of galaxies. This result also suggests that there may be more than one way to form a galaxy.”
New 2020 data from the Hubble Space Telescope explains the reason behind the missing dark matter in NGC 1052-DF4, which resides 45 million light-years from Earth. Astronomers discovered that the missing enigmatic matter can be explained by the effects of tidal disruption from gravity forces of the neighboring massive galaxy NGC 1035 that are stripping NGC 1052-DF4 of dark matter. while the stars feel the effects of the interaction with another galaxy at a later stage.
The discovery of evidence to support the mechanism of tidal disruption as the explanation for the galaxy’s missing dark matter has not only solved an astronomical conundrum, but has also brought a sigh of relief to astronomers. Without it, scientists would be faced with having to revise our understanding of the laws of gravity.
“This discovery reconciles existing knowledge of how galaxies form and evolve with the most favorable cosmological model,” said Mireia Montes of the University of New South Wales in Australia who led an international team of astronomers to study the galaxy using deep optical imaging. They discovered that the missing dark matter can be explained by the effects of tidal disruption.
Until now, said Montes the removal of dark matter in this way has remained hidden from astronomers as it can only be observed using extremely deep images that can reveal extremely faint features. “We used Hubble in two ways to discover that NGC 1052-DF4 is experiencing an interaction,” explained Montes. “This includes studying the galaxy’s light and the galaxy’s distribution of globular clusters.”
At the end of the day, the nature of dark matter remains a huge unsolved enigma. “The nature of dark matter is one of the biggest mysteries in science and we need to use any related new data to tackle it,” says astronomer Avi Loeb with the Harvard-Smithsonian Center for Astrophysics.
Image credit: top of page shows that dark matter in the universe is distributed as a network of gigantic dense (white) and empty (dark) regions, where the largest white regions are about the size of several Earth moons. Van Waerbeke/Heymans/CFHTLens collaboration.