A Galaxy’s Tail

NASA, ESA, Ralf Crawford (STScI)

The Large Magellanic Cloud is a small galaxy, just a tenth of the Milky Way’s mass. It is about 160,000 light years away, which is remarkably close in cosmic terms. In the southern hemisphere it spans the width of 20 Moons in the night sky. While the galaxy seems timeless and unchanging to our short human lives, it is, in fact, a dynamic system undergoing a near collision with our galaxy. Now astronomers are beginning to observe that process.¹

The LMC is unusual for a dwarf galaxy because it’s unusually dense. Based on stellar motion within the LMC, it appears to have a rather small halo surrounding it. This has led some astronomers to argue that the galaxy is not in orbit around the Milky Way. Instead, it is simply passing our galaxy, having made its closest approach. As the galaxy passed through the large and relatively dense halo of the Milky Way, some of the LMC halo would have been stripped away, trailing behind it in a diffuse tail. It’s a likely scenario, but proving it has been a difficult challenge. The halo of the Large Magellanic Cloud is too dark and diffuse for us to observe directly. But this new study has finally observed the LMC halo thanks to some distant quasars.

Mishra, et al

Quasars are powerful beacons powered by supermassive black holes in distant galaxies. Though they are billions of light-years away, their light can be easily observed by radio telescopes and space telescopes such as the Hubble. Using Hubble data, the team looked for quasars in locations where the LMC halo was likely to be. In this way, the light of those quasars would pass through the halo before reaching us, and some of the quasar light would be absorbed by the halo. By measuring the spectra of 28 quasars in the LMC sky region, the team was able to make the first mapping of the small galaxy’s halo. Assuming the LMC had a large halo similar to other small galaxies before its flyby of the Milky Way, the team estimates that the LMC has only held on to about 10% of its original halo. The rest of the halo now streams behind the galaxy like a comet’s tail, though that has yet to be observed.

In the future, the team would like to use more quasars to further map the LMC halo, particularly in the front region where the halo is directly colliding with that of the Milky Way. Such work will help us better understand what happens when galaxies interact and how that can affect the evolution of those galaxies.