The outer reaches of the Milky Way are a different place. Stars are much harder to come by because most of this “galactic halo” is made up of empty space. However, scientists theorize that there is an abundance of one thing in this barren area – dark matter. Now, a team from Harvard and the University of Arizona (UA) spent some time studying and modeling one of the galaxy’s closest neighbors to find out more information about this dark matter and, as a result, came up with an entirely new way to look at yourself the halo on.
The neighbor they used is the Large Magellanic Cloud (LMC), a Milky Way satellite galaxy made up of billions of stars. It is positioned so that it swims around the outer area of the halo and creates a “wake” there through the outer area of the Milky Way, much like a boat creates a wake when it moves through water.
Video discussing how the LMC and other clusters can tell us about dark matter by UT employee Paul Sutter.
Photo credit: Paul M. Sutter YouTube Channel
Given the lack of normal matter in the halo, the wake is created by dark matter that interacts with the universe only through the influence of gravity. The UA and Harvard teams tracked the LMC’s progress through the halo and were able to see an outline of the dark matter wake using a tool they created – the first detailed star map of the outer atrium.
This map required some ingenious investigation to determine which stars were actually separated from the Milky Way or the LMC. The team used a two-step approach by first analyzing data from Gaia, which is able to pinpoint the positions of stars but could not pinpoint their distance, and combining them with data from NEOWISE, which is a specific type of giant stars at this position examined data that helped them determine the distance.
Simulation of how dark matter surrounding the Milky Way is affected by the LMC.
Photo credit: NASA / JPL-Caltech / NSF / R. Hurt / N. Garavito-Camargo and G. Besla
The resulting star map starts about 200,000 light years from the center of the Milky Way and continues to about 325,000 light years above. This strip of the outer halo is also the same area the LMC is moving through, and the Harvard team that originally developed the map contacted the UA team, which is separately developing a model to predict the appearance of dark matter in the galactic halo had developed.
The combined team found that one of the UA models accurately predicted the scattering of the stars in the map developed by the Harvard team. UA’s model used the popular dark matter theory known as “cold dark matter,” and while it seemed to fit the star profile pretty well, there was room for improvement. The UA team continues to optimize the model to see if it fits the observed star pattern better.
UT video describing the hunt for dark matter.
One result of the combined model and star map is more information about the LMC itself. It appears that the LMC is just completing its first orbit around the Milky Way, having formed in the M31 galaxy more than 13 billion years ago. At some point it will collapse into the Milky Way itself, although after a few billion years of the spiral it is wound around it.
This dance provides insight into the general merging of galaxies, and the combined model and map seem to confirm the general theory of how these mergers take place. With a better understanding of the dark matter effects this paper offers, it will be even easier to model these gigantic galactic fusions better than ever.
Learn more:
JPL – Astronomers release new all-sky map of the outer reaches of the Milky Way
Nature – All-Sky-Dynamic Response of the Galactic Halo to the Large Magellanic Cloud
UA astrophysicists help map the invisible “ocean” of dark matter
ScienceDaily – The Milky Way has a lumpy halo
UT – Decaying Dark Matter should be visible here in the Milky Way as a halo around the galaxy
Mission statement:
Star map showing the Milky Way Galaxy and LMC with lighter sections of denser stars, while darker sections are sparse areas of stars.
Photo credit: NASA / ESA / JPL-Caltech / Conroy et al
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