Our Milky Way galaxy doesn’t just hover in space all by itself. There are a number of small satellite galaxies orbiting it like moons around a planet. Two of these galaxies, the Large and Small Magellanic Clouds, have been at the heart of an astronomical mystery for the last 50 years. It was 1965 when scientists discovered a massive stream of gas extending from these two galaxies, but were at a loss to explain its formation. Massive doesn’t actually do the so-called Magellanic Stream justice — it wraps itself halfway around the Milky Way, and is a staggering 600,000 light years long.
Astronomers have posited a variety of hypothesis to explain the presence of such a gigantic cloud of gas trailing off from two small galaxies in the decades since its discovery. Most of these scenarios had the Magellanic Stream forming all at once, possibly as the result of some gravitational interaction between the Small Magellanic Cloud and the Milky Way. It took detailed observation by the Hubble Space Telescope to figure it out.
A team of astronomers, led by Andrew J. Fox of the Space Telescope Science Institute in Baltimore, used Hubble to gather data indicating that the Magellanic Stream is composed of material from both the Large and Small Magellanic clouds (the bright spots in the image above). Perhaps more unexpectedly, it was stripped from the dwarf galaxies at different times.
The astronomers made their observations of the Magellanic Stream by pointing Hubble at quasars — the super-bright active cores of distant galaxies. These were used to backlight the cloud and allowed a precise absorbance spectrum to be gathered. The levels of oxygen and sulfur in different parts of the stream gave researchers the data to pinpoint the age and origin of the gas.
We now know the stream initially formed more than two billion years ago when a stream of matter was ripped from the Small Magellanic Cloud. The part of the gaseous ribbon closer to the dwarf galaxies had a different profile, though. The team concluded that it matched the composition of the Large Magellanic Cloud, which means that part of the stream was formed more recently.
The new research also clarifies the mechanism of formation. As the satellite galaxies are pulled closer to the Milky Way they encounter the halo of hot gas around it, which displaces the gas native to the Magellanic Clouds. The stream is the result of that effect combined with the gravitational tug-of-war between the two smaller galaxies. The displaced gas is simply catapulted off in a gravitational slingshot.
A great deal of effort has been devoted to understanding the Magellanic Stream and the Clouds that produced it. The Large and Small Magellanic Clouds are unique among the Milky Way’s satellite galaxies because they have been able to retain most of their gas envelopes and are still actively forming stars. Without the presence of sufficiently dense gas clouds, star formation stops. This is what we see in most nearby dwarf galaxies, and it gives us a preview of what will one day happen in the Milky Way.
The gas stripped away in the Magellanic Stream is slowly spiraling in toward the Milky Way, and could kick off a wave of new star formation when it gets here. The team believes the stream could get even larger if another puff of gas from the Large or Small Magellanic Cloud were to be thrown off. Even if 600,000 light years is all the longer this stream of star-birthing gas gets, it’s going to be a bright future for our galaxy.
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