Take a trip to one of Disney’s resorts in California or Florida, and you could find yourself at Galaxy’s Edge. Well, Star Wars’ Galaxy Edge at least. The edge of our galaxy is far, far away. A new estimate, featured on the pre-print website arXiv, has found that the Milky Way spans approximately 1.9 million light-years in diameter, more than seven times wider than previous estimates.
Last year, GAIA mapping data provided a rough disk diameter of 258,000 light-years. However, astronomers acknowledged that there was a great big problem lurking in the outer reaches of the galaxy that makes these estimations problematic. Dark matter.
A vast halo of dark matter appears to engulf the visible matter in the Milky Way’s disk and reach far beyond it as well. Given that we can’t see it, the Milky Way’s dark halo is a pretty hard thing to measure. However, astronomers can infer its presence by studying the motion of cosmic objects around it, which is exactly how this latest estimate came about.
Dr Alis Deason, from Durham University, and her colleagues produced computer simulations of the dark matter haloes of galaxies of a similar mass to the Milky Way. Not only were these mapped in isolation but also in situations where the galaxy was close to another – analogous to the Milky Way and our nearest giant neighbor Andromeda. In these instances, the velocities of other small nearby galaxies, included in the simulations to mimic the dwarf galaxies around the Milky Way in the Local Group, dropped sharply at the edge of the giant galaxy’s dark halo.
Stepping away from the simulations, the team looked at telescope observations of the actual dwarf galaxies in the Local Group around our galaxy and found a similar drop in their radial velocities, indicating the Milky Way’s dark halo boundary. This occurred at around 950,000 light-years from our galaxy’s center – 35 times farther from the galactic center than the Sun is.
“In many analyses of the Milky Way halo its outer boundary is a fundamental constraint,” wrote the study author’s in their paper. “Often the choice is subjective, but as we have argued, it is preferable to define a physically and/or observationally motivated outer edge.”
Going forward, the team hope that their data can help others generate more robust and accurate measurements of the edge of the Milky Way, as well as potentially aid the discovery of additional small galaxies nearby.