Dark matter acts like cosmic glue, accumulating in "halos" in which galaxies form. It makes up about 85% of all the matter in the cosmos, meaning all the stars and other celestial objects we see account for a small fraction of the universe's mass. And as astronomer Vera Rubin observed, despite its invisible nature, dark matter seems essential to explain the apparent movement of stars around their galactic centers.
"The most surprising aspect is that this finding indicates that an entire trail of galaxies including DF2, DF4, and DF9 formed together in an extreme event producing galaxies not alike any we've ever seen before — the first of its kind," Michael Keim, an astrophysicist at Yale's Graduate School of Arts and Sciences and the study's first author, told Live Science via email. "As a result, they have a ghostly appearance, bright clusters of stars, and a lack of dark matter."
To calculate the mass of DF9, the researchers measured the motion of its stars by analyzing changes in their wavelengths. The analysis revealed that DF9 has a measly mass of approximately 100 million suns, which is consistent with its visible mass in stars. If it contained the expected amount of dark matter, it would be exponentially heavier, harboring a mass of more than 10 billion suns, the researchers said in a statement.
A video showing the galactic collision that resulted in this string of dwarf galaxies. (Image credit: Video courtesy of W. M Keck Observatory / Adam Makarenko)
"If DF2, DF4, and DF9 were the only galaxies known, one would not conclude there is any need for dark matter," the researchers wrote in the paper. These three galaxies do not display a mass-based discrepancy based on their visible and invisible matter; their mass, determined by the gravitational motion of their stars, matches their mass as measured by the light emitted from these same stars.
Dark matter deficiency is linked to a violent past
The team likened this lineup of dwarf galaxies to a miniature version of the Bullet Cluster, located nearly 4 billion light-years from Earth. The collision of two galaxy clusters that created the Bullet Cluster is considered the most energetic event known since the Big Bang, astronomers say, and the massive mashup is responsible for separating the system's gas from its dark matter.
The Bullet Cluster seen by the Chandra Telescope (Image credit: (X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.))"The Bullet Dwarf event in which DF9 is thought to have formed is named after the famous Bullet Cluster wherein the gas of two entire clusters of galaxies was separated out in a similar manner as this 'Bullet Dwarf' collision, so indeed this can occur at much larger scales," Keim explained via email. "But collisions between giant clusters are far less frequent than collisions of individual galaxies."
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The strange separation between dark matter and visible matter in the galaxies of the NGC 1052 field warrants further study, the researchers said. Finding any leftover gas from the collision would provide the "smoking gun" evidence for the collisional origin of this faint string of galaxies.
The astronomers are conducting follow-ups with other observatories, including the new MOTHRA facility, the "world's largest all-lens telescope," which is under construction in Chile. With an array of 1,140 telephoto lenses, "its line mapping sensitivity on large scales — like that of the trail — will be at least an order of magnitude deeper than any other facility," Keim added.
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