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.
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.
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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.
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.
The study was published in The Astrophysical Journal.