Astronomers Solve Mystery of Magellanic Stream’s Origin

Scientists have found that most of the Magellanic Stream was stripped from the Small Magellanic Cloud about 2 billion years ago, with a smaller portion originating more recently from the Large Magellanic Cloud.

In this composite image, the Magellanic Stream is shown in pink. The Milky Way is the light blue band in the center of the image. The Magellanic Clouds, satellite galaxies of the Milky Way, are the white regions at the bottom right (David L. Nidever et al. / NRAO / AUI / NSF / Mellinger / Leiden / Argentine / Bonn Survey / Parkes Observatory / Westerbork Observatory / Arecibo Observatory)

The Small Magellanic Cloud and the Large Magellanic Cloud are two dwarf galaxies orbiting our Milky Way Galaxy. They are at the head of a huge gaseous filament known as the Magellanic Stream.
Since the discovery of the stream in the early 1970s, astronomers have wondered whether this gas comes from one or both of the satellite galaxies.
Now, new Hubble observations show that most of the gas was stripped from the Small Magellanic Cloud about two billion years ago – but surprisingly, a second region of the stream was formed more recently from the Large Magellanic Cloud.
The team used Hubble’s Cosmic Origins Spectrograph (COS) and ESO’s Very Large Telescope to measure the abundances of heavy elements, such as oxygen and sulfur, at six locations along the Magellanic Stream.
Hubble’s COS detected these elements from the way they absorb the ultraviolet light released by faraway quasars as it passes through the foreground Stream.
The scientists found low abundances of oxygen and sulfur along most of the stream, matching the levels in the Small Magellanic Cloud about two billion years ago, when the gaseous ribbon was thought to have been formed.
In a surprising twist, the team discovered a much higher level of sulfur in a region closer to the Magellanic Clouds. This discovery was unexpected – computer models of the Stream predicted that the gas came entirely out of the Small Magellanic Cloud, which has a weaker gravitational pull than its more massive cousin.
All of the Milky Way’s nearby satellite galaxies have lost most of their gas content — except the Magellanic Clouds. As they are more massive than these other satellites they can cling on to this gas, using it to form new stars. However, these Clouds are approaching the Milky Way and its halo of hot gas. As they drift closer to us, the pressure of this hot halo pushes their gas out into space. This process, together with the gravitational tug-of-war between the two Magellanic Clouds, is thought to have formed the Magellanic Stream.
“Exploring the origin of such a large stream of gas so close to the Milky Way is important,” said Dr Andrew Fox from the Space Telescope Science Institute, who is a co-author of two papers published in the Astrophysical Journal (paper1 and paper2).
“We now know which of our famous neighbors, the Magellanic Clouds, created this gas ribbon, which may eventually fall onto our own galaxy and spark new star formation. It’s an important step forward in figuring out how galaxies obtain gas and form new stars.”

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