The edge of the Milky Way has been located (and it's much further than we thought)

Astronomers have long known that the brightest part of the Milky Way, the disc of oval-shaped stars that houses the Sun, is about 120,000 light-years in diameter. Beyond this star disc is a disc of gas: a vast halo of dark matter that envelops both disks and extends far beyond them. However, since the dark halo does not emit light, it is difficult to elucidate its diameter. Now, thanks to a new study led by astrophysicist Alis Deason of the University of Durham in England, he has managed to define the edge of the Milky Way: it spans almost two million light years, more than 15 times wider than its spiral luminous disc, which proves, again, that our galaxy is much larger than it appears.

Experts used nearby galaxies to locate the edge of the Milky Way, pinning the diameter at 1.9 million light-years, more than seven times wider than previous estimates, according to their article published in arXiv.org. The edge of our galaxy is far, far away.

This new figure could lead to a better estimate of how massive the galaxy is and how many galaxies orbit it in detail.

Problems in calculating its diameter

GAIA data provided an approximate disk diameter of 258,000 light years in 2019. However, astronomers recognized that there was a big problem lurking at the edge of the galaxy that could be giving us the wrong information: dark matter. As we have commented, a huge halo of dark matter seems to envelop visible matter in the disc of the Milky Way and beyond (just as the gravitational influence of the Sun extends beyond the Kuiper belt). Even if we can’t see it - to dark matter - astronomers can infer its presence by studying the movement of cosmic objects around it, how it affects everything around it, which is exactly how this last estimate came about.

First, they performed high-resolution cosmological simulations of the dark matter halos of galaxies of the same mass as the Milky Way, both in isolation and in their analogs from the Local Group, a small group of galaxies of approximately 9,8 million light-years across, to which our own belongs.

They focused especially on M31, also known as the Andromeda galaxy, the galaxy that the Milky Way will collide with in about 4.5 billion years. Both are currently separated by approximately 2.5 million light-years, close enough to gravitationally interact.

Computer simulations

Using several different simulation programs, the researchers modelled the dark matter halo of the Milky Way, observing the radial velocity, the orbital velocity of objects moving around the galaxy at various distances, and density to try to define the edge of the dark matter halo. Simulations showed that just beyond the edge of the dark halo of a giant galaxy, the velocities of small nearby galaxies were falling sharply.

"The edge of the stars is very sharp, almost as if the stars stopped at a particular radius," Deason explains.

The radial distance was about 292 kilometres, approximately 950,000 light years from the center of our galaxy, 35 times farther from the galactic center than the Sun. If we double it, we get the diameter and, therefore, just over 1.9 million light years, the desired figure.

This new data should also help astronomers discover other galactic properties; for example, the larger the Milky Way, the more massive it is, and more galaxies should revolve around it.

Reference: A. J. Deason et al. The edge of the galaxy. arXiv:2002.09497. Posted February 21, 2020.