A fresh view on our galaxy’s place in the universe has emerged from high-resolution simulations. Unlike the traditional view of galaxies suspended in a round halo, the Milky Way is now seen as part of a vast, flat structure mainly made of dark matter.
This new perspective addresses a long-standing puzzle: why nearby galaxies seem to move more slowly than expected. Previous models suggested a spherical mass distribution but couldn’t explain the discrepancies between theory and observations.
Researchers have now identified a dark matter sheet, stretching millions of light-years, that may be driving these unexpected movements. This newly proposed structure not only matches the speeds of nearby galaxies but also aligns with how galaxies are spread across space.
Published in Nature Astronomy, the study, led by Ewoud Wempe from the Kapteyn Astronomical Institute, utilized advanced simulation methods to analyze the Milky Way’s surroundings. Their findings indicate a unique, flat distribution of dark matter, a notable shift from earlier assumptions.
The team produced 169 simulations to create a model of the Local Group, which includes the Milky Way and other nearby galaxies. They found a dark matter sheet that expands over 10 megaparsecs (about 30 million light-years). This sheet has a higher density than average, which shapes how gravity works in the area.
The way this structure influences gravitational pulls is significant. Mass in the same plane can create outward forces, slowing down the motion of galaxies toward the center. This explains why galaxies glide more smoothly than we’d expect from a spherical model.
Interestingly, the dark matter sheet closely follows the Supergalactic Plane, a structure defined by visible galaxies, suggesting that visible matter traces this larger, invisible framework.
For decades, astronomers estimated the Local Group’s mass using the timing argument, treating the Milky Way and the nearby Andromeda galaxy as a simple two-body system. These estimates, starting in 1959, often conflicted with real galaxy motion data. Most models assumed a spherical mass and reported masses between 1.3 and 2.3 trillion solar masses for the Local Group. Yet, these figures couldn’t clarify the slow movement of galaxies in the outskirts.
The new sheet model resolves this. It suggests the Local Group has about 3.3 trillion solar masses, but the larger dark matter sheet could be four times that. This model matches galaxy velocities more accurately than traditional approaches.
The predicted motion within this sheet aligns with measurements, showing speeds typically under 30 kilometers per second, consistent with the expected calm of the local universe. However, as galaxies stray away from this midplane, their speeds increase, reflecting a complex gravitational landscape.
The concept of dark matter sheets isn’t exclusive to the Milky Way. Observations from the Atacama Large Millimeter/submillimeter Array (ALMA) indicate that massive galaxies formed in densely packed dark matter regions during the universe’s early days. For instance, SPT0311-58, discovered in 2017, consists of two galaxies that existed just 780 million years after the Big Bang, embedded in a massive dark matter halo.
This resemblance supports the idea that such dark matter sheets could significantly impact how galaxies form and move. They might have been crucial not just in our time but also in shaping the cosmos’s early structure.
While these simulations provide solid data, the model does have some limitations. Most galaxy observations used in the study are near the Supergalactic Plane, which makes it hard to gauge dynamics from surrounding areas. Predictions suggest that there should be strong influences from above and below the dark matter sheet, with some speeds exceeding 100 kilometers per second. However, this remains untested due to a scarcity of high-latitude observations.
Going forward, identifying more isolated dwarf galaxies in these regions could validate the dark matter sheet concept. Although the simulations are confined to a 40-megaparsec space, discoveries in these areas could reshape our understanding of galactic dynamics and the universe as a whole.
For further insights on the early universe, ALMA’s discoveries can be explored in reports from Phys.org, highlighting the ongoing quest to understand the universe’s complex structure.
