Edge of Milky Way Uncovered

By: Jonathan Xu

The size of the entire Milky Way has remained a mystery until in February, when an estimate for the size of our galaxy was discovered by Alis Deason, whose research revealed it to be fifteen times wider than the luminous disk we see in the sky.

The bright disk that makes up the visible part of the Milky Way is estimated to be approximately 120,000 light years (a measure of distance that is equal to how far light can travel in a year) across. The Milky Way has a massive halo around it that is mostly dark, which makes it hard to determine a measurement of length. In the end of her research, Alis Deason concluded that the outer-limits of our galaxy is 1.9 ± 0.4 million light years or 292 ± 61 kpc (kiloparsec), which is truly a spectacular number. To put this into perspective, this is approximately 35 times further from the center of the Milky Way than the sun is, even though the sun is considered to be on the far side one of the galaxy’s longer arms.

Alis Deason published a paper on February 21, writing, “We use cosmological simulations of isolated Milky Way-mass galaxies, as well as Local Group analogues, to define the ‘edge’ — a caustic manifested in a drop in density or radial velocity — of Galactic-sized haloes, both in dark matter and in stars.” This excerpt from Alis Deason’s research paper begins to tell how she used computer simulations of how galaxies similar to the Milky Way form to find out more about these far-out locations in the galaxy.

During her research, Alis discovered that there are still some stars out in the dark outer reaches of the Milky Way. “Both [dark matter and stars] have a well-defined edge,” Deason says. “The edge of the stars is very sharp, almost like the stars just stop at a particular radius.” By edge, Deason means how far a star (or dark matter) can be from the center of the galaxy. She talks about the revelation that there is almost a clear and defined border, which stars cannot exist outside of.

The result of this answer could potentially help many researchers to determine more about the Milky Way in future astronomical research about the galaxy. The size of our galaxy can be used to determine certain gravitational forces that influence the movement of smaller galaxies, particularly the ones within the same local group, or a large group of galaxies that will not be separated by the infinite expansion of spacetime. Ultimately, this discovery can lead the astronomical community to discover many new celestial bodies beyond the galaxy.

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