Astronomers have solved a 40-year mystery on the origin of Magellanic Stream - a ribbon of gas stretching almost halfway around our Milky Way galaxy.
The Large and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way, are at the head of the gaseous stream.
Since the stream's discovery by radio telescopes in the early 1970s, astronomers have wondered whether the gas comes from one or both of the satellite galaxies.
New Hubble observations reveal most of the gas was stripped from the Small Magellanic Cloud about 2 billion years ago, and a second region of the stream originated more recently from the Large Magellanic Cloud.
Astronomers, led by Andrew J Fox of the Space Telescope Science Institute in Baltimore, determined the source of the gas filament by using Hubble's Cosmic Origins Spectrograph to measure the amount of heavy elements, such as oxygen and sulphur, at six locations along the Magellanic Stream.
They observed faraway quasars, the brilliant cores of active galaxies, that emit light that passes through the stream. They detected the heavy elements from the way the elements absorb ultraviolet light.
Fox's team found a low amount of oxygen and sulphur along most of the stream, matching the levels in the Small Magellanic Cloud about 2 billion years ago, when the gaseous ribbon is thought to have formed.
In a surprising twist, the team discovered a much higher level of sulphur in a region of the stream that is closer to the Magellanic Clouds.
This discovery was a wrinkle Fox's team didn't expect, because computer models of the stream predicted that the gas came entirely out of the Small Magellanic Cloud, which has less gravity than its more massive cousin.
Astronomers have debated whether the two Magellanic Clouds are on their first pass near our Milky Way or are bound to it.
"What's interesting is that all the other nearby satellite galaxies of the Milky Way have lost their gas. The Magellanic Clouds have been able to retain their gas and are still forming stars because they're more massive than the other satellites," Fox said.
"However, as they're now approaching the Milky Way, they're feeling its gravity more and also encountering its halo of hot gas, which puts pressure on them. That process, together with the gravitational tug-of-war between the Magellanic Clouds, leads to the production of the stream. You're seeing material stripped out of the Clouds as they come in toward the Milky Way," said Fox.
The study was published in The Astrophysical Journal.
