New All-Sky Map of the Milky Way’s Galactic Halo

The outer reaches of the Milky Way galaxy are a different place.  Stars are much harder to come by, with most of this “galactic halo” being made up of empty space.  But scientists theorize that there is an abundance of one particular thing in this desolate 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 nearest neighbors to try to tease out more information about that dark matter, and as a result came up with an all new way to look at the halo itself.

The neighbor they used is the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way made up of several billion stars.  It is positioned such that it is floating around the outer reaches of the halo where it creates a “wake” through the Milky Way’s outer reaches, similar to how a boat creates a wake when it travels through water.

Given the paucity of normal matter in the halo, the wake is made through dark matter, which interacts with the universe only through the influence of gravity.  Tracking the progress of the LMC through the halo, the UA and Harvard teams were able to discern an outline of the dark matter wake by utilizing a tool they created – the first ever detailed star map of the outer halo.

That map required some inventive sleuthing to determine which stars were actually separate from the Milky Way or the LMC. The team used a two tiered approach by first analyzing data from Gaia, which is able to accurately pinpoint stars locations but couldn’t tell their distance, and combining it with data from NEOWISE, which looked at a specific type of giant star in that location data that helped them determine the distance. 

 The resulting star map starts about 200,000 light years away from the center of the Milky Way and continues to about 325,000 light years beyond it.  This swath 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 who had separately come up with a model for predicting how dark matter would look in the galactic halo.

The combined team found that one of UA’s models accurately predicted the dispersion of stars in the map the Harvard team had developed.  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 some room for improvement. The UA team is continuing to tweak the model to see if they can get a better fit to the observed star pattern.

One outcome 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 after being formed in the M31 galaxy more than 13 billion years ago.  Eventually it will collapse into the Milky Way itself, though after another few billion years of spiraling around it. 

That dance offers insight into how galaxies merge more generally, and the combined model and map seem to confirm the general theory of how those mergers happen.  With a better understanding of the effects of dark matter that this paper provides it will become even easier to model these gigantic galactic fusions better than ever before.

Learn More:
JPL – Astronomers Release New All-Sky Map of Milky Way’s Outer Reaches
Nature – All-sky dynamical response of the Galactic halo to the Large Magellanic Cloud
UA – Astrophysicists Help Chart Dark Matter’s Invisible ‘Ocean’
ScienceDaily – The Milky Way galaxy has a clumpy halo
UT – Decaying Dark Matter Should be Visible Here in the Milky Way as a Halo Around the Galaxy

Lead Image:
Star map showing the Milky Way and LMC with lighter sections showing more dense stars while darker sections are more sparse areas of stars.
Credit: NASA / ESA / JPL-Caltech / Conroy et al