This is a photo mosaic of an edge-on view of the Milky Way galaxy, looking toward the central bulge. Superimposed on it are radio-telescope images, colored pink, of the stretched, arc-shaped Magellanic Stream below the plane of the galaxy and the shredded, fragmented Leading Arm crossing the galaxy's plane and extending above it. These gas clouds are being gravitationally pulled apart like taffy from the Small and Large Magellanic Clouds - satellite galaxies to our Milky Way - which appear as bright clumps within the gas. The source of the ribbon-like Magellanic Stream was uncovered by the Hubble Space Telescope about five years ago, and it was found to come from both Magellanic Clouds. However, the source of the Leading Arm remained a mystery until today. Now, scientists have used Hubble's ultraviolet vision to chemically analyze the gas in the Leading Arm and determine its origin. Because they could not directly sample it, they instead used the light from seven quasars - the bright cores of active galaxies - to measure how it filtered through the Leading Arm's gas. These quasars reside billions of light-years beyond the Leading Arm and act as "lighthouses" shining through the gas.Scientists looked for the absorption of the quasars' ultraviolet light by oxygen in the cloud. This is a good indication of how many heavier elements reside in the Leading Arm's gas. The team then compared Hubble's measurements to hydrogen measurements made by the Robert C. Byrd Green Bank Telescope in Green Bank, West Virginia, as well as several other radio telescopes. Marked locations indicate the three brightest of the seven quasars used to study thecomposition of the Leading Arm. Spectra for these three quasars are superimposed at the bottom of the graphic. The vertical axis of each spectrum indicates how much absorption is taking place. The more absorption, the greater the signal strength is. The horizontal axes indicate the velocities of the gas at different points. Blue boxes is

Hubble solves Magellanic mystery

Observations with the Hubble Space Telescope have seemingly solved a galactic mystery: the source of an arch of cosmic gas connecting two dwarf galaxies that orbit the Milky Way.

Try out a subscription to BBC Sky At Night Magazine and pay just £9.99 for 6 issues today!

Ezzy Pearson

Published: March 26, 2018 at 11:00 am

The 'Leading Arm' is the name given to an arch of cosmic gas that connects two dwarf galaxies known as the Large Magellanic Cloud and the Small Magellanic Cloud.

These two galaxies orbit our own Galaxy, but also orbit each other, and the tug of war between the two has produced this connecting structure, which is thought to be as old as two billion years.

The gas in the Leading Arm is being ‘eaten’ by the Milky Way, generating the birth of new stars in our Galaxy, but which of the two dwarf galaxies is doing the tugging?

Andrew Fox of the Space Telescope Science Institute has been looking into the mystery.

ALMA_Chile — The Large and Small Magellanic Clouds can be seen from the southern hemisphere under dark skies. They appear as fuzzy blobs in the centre and the upper right of this image, taken at the Atacama Large Millimeter/submillimeter Array in the deserts of northern Chile. Credit: ESO/C. Malin

Using the Hubble Space Telescope’s ultraviolet capabilities, he and his team were able to chemically analyse the gas in the Leading Arm.

They looked at seven quasars, which are the bright cores of active galaxies that are billions of lightyears away from the cloud.

They measured how the light filtered through the cloud; in particular the absorption of ultraviolet light by oxygen and sulphur.

“We’ve found that the gas matches the Small Magellanic Cloud,” says Fox.

“That indicates the Large Magellanic Cloud is winning the tug-of-war, because it has pulled so much gas out of its smaller neighbour.”

The process that creates the Leading Arm is a well-known phenomenon called ‘gas accretion’ in which gas falls into galaxies.

Normally this is a difficult process to observe in galaxies because they are so far away, but not on this occasion.

“As these two galaxies are in our backyard, we essentially have a front-row seat to view the action,” says Kat Barger of Texas Christian University, a collaborator in the study.

Science journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.