In the process of charting the movements of nearby galaxies, researchers at the Leibniz Institute for Astrophysics have reached a curious conclusion: There's a narrow band of dark matter extending from our Local Group of galaxies, which includes the Milky Way, to the Virgo Cluster some 65 million light-years away. On either side of it are just bubbles of nothingness.
The revelation, which is described in the current Monthly Notices of the Royal Astronomical Society, could help explain a lingering mystery relating to the unusual distributions of dwarf galaxies often found scattered just beyond the boundaries of galaxies like the Milky Way. Rather than the expected random distribution, like bees buzzing around a hive, the dwarves seemed to be confined to a flat plane or disc of space. In the case of the Andromeda galaxy, that plane is nearly a million light years across but only 30,000 light years thick. A stellar pancake.
This is unexpected. The arrangement surely isn't random, but the dwarf galaxies should be orbiting independently of each other. Simulations have reached this same conclusion: bees buzzing.
So, there is an unknown at work. Current theories don't account very well for an anisotropic (directionally dependent) distribution of dwarf galaxies.
"Although not identically defined, the evidence for the ubiquitous anisotropic distribution of satellite galaxies has by now a strong observational foundation," the current paper notes. "The abundance of evidence pointing towards the anisotropic distribution of satellites around large central host galaxies calls for a comprehensive and robust explanation, within the framework of the cold dark matter with dark energy (ΛCDM) model." And this is where we find our intergalactic dark matter bridge.
"This is the first time we have had observational verification that large filamentary super highways are channeling dwarf galaxies across the cosmos along magnificent bridges of dark matter."
More generally, what's needed to explain the satellite disc distributions is some variety of asymmetry. If the gravitational forces surrounding the Milky Way are all about the same, on average, we'd expect the aforementioned bees. But if something were out there exerting some additional force, it's easier to imagine such an extreme flattening. This is where a dark matter bridge starts to make sense: an additional outside force to begin the process of structure formation, a seed for anisotropy. Once evenly distributed galaxies begin to align around a single axis.
It's not all that weird really. The distribution of stuff across the universe is on very large scales evenly spread around, but on smaller scales is more of a collection of galaxy clusters surrounded by deep voids. These voids are bridged by thin filaments of material, which are the dark matter bridges in question.
In the paper's words: "The analysis reveals that the Local Group and Centaurus A reside in a filament stretched by the Virgo cluster and compressed by the expansion of the Local Void. Four out of five thin planes of satellite galaxies are indeed closely aligned with the axis of compression induced by the Local Void."
In a statement, Noam Libeskind, an astrophysicist and the study's lead author, puts it a bit more poetically. "This is the first time we have had observational verification that large filamentary super highways are channeling dwarf galaxies across the cosmos along magnificent bridges of dark matter," he says. The filaments offer dwarf galaxies an "off ramp" of sorts along which they can be beamed towards the Milky Way and other galaxies sporting similar satellite accretion discs. "The fact that this galactic bridge can affect the dwarf galaxies around us is impressive, given the difference in scale between the two: the planes of dwarfs are around 1 percent of the size of the galactic bridge to Virgo."
The current study can also be read in free, open-access form on the arXiv preprint server.