The image above shows detail of the Leading Arm, an enormous stream of gas and dust that connects the Milky Way and its two closest satellite galaxies, the Large and Small Magellanic Clouds. Note the locations of the two galaxies within the stream of gas and dust relative to the plane of the Milky Way, which is represented by the dark band across the center of the main image.
It has long been known that the two Magellanic Clouds orbit each other, and that huge volumes of dust and gas are being pulled off both galaxies by each other and the Milky Way. What was not known with certainty, however, was which of the two smaller galaxies was the main contributor to the bridge of material known as the Leading Arm, which is thought to be between one and two billion years old.
At first glance, it might appear as though the stream of material originates in the Large Magellanic Cloud, but since the Leading Arm is the closest example of mass transfer between galaxies in real time, astronomers wanted to know which of the two satellite galaxies actually contributed the bulk of the material that is being transferred to the Milky Way. To answer this question required a new approach to the problem, since Earth’s atmosphere absorbs or scatters most of the light from the Leading Arm that could have been used to analyze the nature and composition of the stream of material that forms the bridge.
The question to answer was this; does the stream of dust and gas have the composition of the Large Magellanic Cloud (LMC), or does it have the composition of the Small Magellanic Cloud (SMC)? However, the biggest problem to finding an answer was that while it was known that both galaxies had contributed to the bridge of material, on the other hand, a large degree of mixing had taken place which obscured the issue somewhat. Nonetheless, a research team led by Andrew Fox of the Space Telescope Science Institute in Baltimore, hit on the idea of using the Hubble Space Telescope’s Cosmic Origins Spectrograph to look for UV absorption lines of particularly sulphur and oxygen, since these lines in UV light cannot be observed from Earth.
Essentially, Fox and his team identified seven quasars located on the far side of the Leading Arm, and looked for the UV absorption lines of oxygen and sulphur as the light from the quasars filtered through the structure of the Leading Arm, since these particular absorption lines are excellent indicators of how many other heavy elements are present in the material being analyzed.
Comparing this data with hydrogen measurements of the same part of the Leading Arm made previously by the National Science Foundation’s Robert C. Byrd Green Bank Telescope at the Green Bank Observatory in West Virginia and several other radio telescopes, Fox and his team found that the overall composition of the Leading Arm matched that of the Small Magellanic Cloud, thereby solving a seventy-year old mystery.
With the clarity that perfect hindsight brings, it is now clear why the SMC would be the predominant source of the material in the Leading Arm. Since the LMC is the larger of the two galaxies, it is obvious that it would pull more material from the SMC than the SMC would pull off, or out of the LMC. The Milky Way on the other hand, is more massive than the two smaller galaxies combined, so it is not surprising that it would capture all of the material from both galaxies.
Nonetheless, the front end of the Leading Arm has now reached the Milky Way’s disc, where it serves as both the source of material and the triggering mechanism for new star formation, which means that at some point in the future, new life may emerge on planets whose origins lie outside of the Milky Way.