Astronomers have long known that the brightest part of the Milky Way, the oval-shaped disk of stars that houses the Sun, is about 120,000 light-years across . Beyond this stellar disk is a disk of gas: a vast halo of dark matter that envelops both disks and extends far beyond them. But since the dark halo does not emit light, it is difficult to determine its diameter. Now, thanks to a new study led by astrophysicist Alis Deason of the University of Durham in England, it has managed to define the edge of the Milky Way: it extends for almost 2 million light years, more than 15 times wider than its disk. luminous in a spiral, which shows, again, that our galaxy is much larger than it seems.
The experts used nearby galaxies to locate the edge of the Milky Way, specifying the diameter at 1.9 million light years, more than seven times wider than previous estimates, according to their article published on arXiv.org. The edge of our galaxy is very, very 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.
The problems in calculating its diameter
The GAIA data in 2019 provided an approximate disk diameter of 258,000 light years. However, astronomers recognized that there was a big big problem lurking at the edge of the galaxy that could be giving us a wrong piece of data: dark matter. As we have discussed, a huge halo of dark matter appears to envelop visible matter in the Milky Way disk and beyond (just as the Sun’s gravitational influence extends beyond the Kuiper belt). Even though we can’t see it – dark matter – astronomers can infer its presence by studying the motion of cosmic objects around them, how it affects everything nearby, which is exactly how this latest 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 Local Group analogs, a small group of galaxies of about 9.8 million. light years in diameter, to which ours belongs.
They especially focused on M31 , also known as the Andromeda galaxy, the galaxy with which the Milky Way will collide in about 4.5 billion years. The two are currently approximately 2.5 million light-years apart, close enough to interact gravitationally.
Computer simulations
Using several different simulation programs, the researchers modeled the Milky Way’s dark matter halo, looking at radial velocity, the orbital velocity of objects moving around the galaxy at various distances, and density to try to define the edge of dark matter halo . The simulations showed that just beyond the edge of a giant galaxy’s dark halo, the speeds of nearby small galaxies dropped sharply.
“The edge of the stars is very sharp, almost as if the stars stopped at a particular radius,” explains Deason.
The radial distance was around 292 kiloparsecs , roughly 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 obtain 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 the more galaxies should revolve around it.
Referencia: A. J. Deason et al. The edge of the galaxy. arXiv:2002.09497. Posted February 21, 2020.
