Back in 1972, radio astronomers discovered vast elongated cloud of gas arcing across the entire sky. Long enough to stretch from horizon to horizon, the only clue as to its origins were that it seemed thickest around the Large and Small Magellanic clouds (LMC and SMC). Over time, increasingly sophisticated computer models suggested that the gas had been dragged out of the Magellanic Clouds by the Milky Way’s gravity after a series of close encounters between the different galaxies. It was assumed that the bulk of the gas had come from the SMC because it’s lower gravity would have given it a weaker grip on its gas clouds. But new spectroscopic data from the Hubble Space Telescope has managed to overturn that idea.The Milky Way galaxy has a number of small satellite galaxies, and the Magellanic Clouds are the largest of those. The LMC and SMC are large enough to carry significant gas reserves, from which new stars can continue to be born. They appear in the sky as hazy islands many times larger than the full moon and looking like seperated sections of the Milky Way. They are only visible in the Southern Hemisphere, and were discovered by the portugese explorer Ferdinand Magellan (or at least, somebody in his party) when he began exploring the southern seas.
The Magellanic Stream lies about 180,000 light years away, and is over 600,000 light years long. From our point of view it spans a full 180° of sky – if it were visible to the naked eye, we could see it stretch all the way from the horizon in one direction, arc across the sky and end back at the horizon in the opposite direction. After it’s discovery in 1972, it took two years for astronomers to show that it was connected in some way to the Magellanic Clouds. Over the next few years, various theories were proposed but it wasn’t until 1980 that computer modelling allowed these ideas to be tested. The early models were extremely simple, taking shortcuts like ignoring the clouds own gravity, or pretending that the cloud and the galaxies were each composed of only a handful of particles. But over time, as computing power grew, the models could be made more complex, to better match reality. By 2010, the most sophisticated models had shown that the stream originated from the SMC, was created by tidal interactions with the Milky Way galaxy, and shaped by the gravity of the nearby LMC.
But recently several astronomers found new way of looking at the problem when they pointed the Hubble Space Telescope at the stream and looked at it through Hubble’s Cosmic Origins Spectrograph (COS). This instrument allowed them to split the incoming light into a spectrum, covering the range of colours right up into the ultra-violet band – far more than the eye can perceive. By measuring the characteristic lines which appear in such a spectrum, astronomers can identify the individual molecules making up a gas. These signatures can then be compared with other nearby objects to find relationships between them. If two objects have the same ratios of different isotopes of common elements, for example, then scientists can conclude that they both have a common origin.
“We’re finding a consistent amount of heavy elements in the stream until we get very close to the Magellanic Clouds, and then the heavy element levels go up,” says Andrew Fox, a staff member supported by ESA at the Space Telescope Science Institute, USA, and lead author of one of two new papers reporting these results. “This inner region is very similar in composition to the Large Magellanic Cloud, suggesting it was ripped out of that galaxy more recently.”
In other words, the bulk of the stream contains low amounts of oxygen and sulphur, and this matches the composition of the SMC. Therefore, most of the stream comes from the SMC and the existing theories are partially correct. But the portion of the stream closest to the Magellanic Clouds has a very different composition, containing relatively high amounts of sulphur which matches the LMC. This directly contradicts the theory, as it was assumed that only the SMC contributed material to the stream. So where does this data lead us?
The new theory is that, about two billion years ago, tidal interactions with the SMC drew out a vast stream of gas, which makes up the bulk of what we see today. This is very similar to what the old theory predicted, but we then had a second event happening much more recently when the LMC drifted close to the Milky Way. Although it’s gravity is strong enough to keep its gas from escaping under tidal stresses, there was a second force at play: The LMC was ploughing through the Milky Way galaxy’s Halo, a huge, thin sphere of hot gas. Ram pressure from this interaction would have given the LMC’s gas clouds enough of an extra kick to help them escape out into space, and that gas forms the second region of the Magellanic Stream.
Interestingly, this discovery would never have been made without the Hubble Space Telescope, which is scheduled to be decommissioned a few years from now. “As Earth’s atmosphere absorbs ultraviolet light, it’s hard to measure the amounts of these elements accurately, as you need to look in the ultraviolet part of the spectrum to see them,” says Philipp Richter of the University of Potsdam, Germany, and lead author on the second of the two papers. “So you have to go to space. Only Hubble is capable of taking measurements like these.”