Hubble solves mystery of gas ribbon around Milky Way
Sen—For 40 years, astronomers have been puzzled by a vast ribbon of gas that is wrapped nearly halfway around our Milky Way galaxy. Now, thanks to the Hubble Space Telescope, the riddle appears solved.
Observations by the orbiting observatory, operated jointly by NASA and the European Space Agency, reveals that the band of material comes from two small companions of our own galaxy.
Most of the stream was pulled away from the Small Magellanic Cloud around two billion years, while a lesser amount came more recently from its larger neighbour, the Large Magellanic Cloud.
These two collections of stars are dwarf galaxies that orbit our Milky Way. They are both placed in the far southern sky, so never rise above the horizon for most of the northern hemisphere.
Though both are easy to spot with the unaided eye in a dark sky, their location means that many amateur astronomers never get to see them. They are named after the 16th century Portuguese explorer Ferdinand Magellan who noted them on his travels.
The lengthy filament of gas, discovered in the early 1970s, is known as the Magellanic Stream and astronomers suspected that it came from one or both of the satellite galaxies.
The confirming evidence came from observations with a Hubble instrument called the Cosmic Origins Spectrograph (COS), along with ground observations made with ESO’s Very Large Telescope.
An observing campaign by two teams of astronomers measured the relative amounts of heavy elements, such as oxygen and sulphur, at six points within the Magellanic Stream. COS detected these elements from the way they absorbed bright ultraviolet light from far more distant galaxy cores, called quasars, as it passed through the ribbon of gas.
Along most of the Magellanic stream were found to be low abundances of oxygen and sulphur which matched the levels that existed in the Small Magellanic Cloud about two billion years ago, when the ribbon is believed to have been formed.
The astronomers got a surprise though when they found a much higher level of sulphur in a region closer to the Magellanic Clouds.
The Large and Small Magellanic Clouds photographed by the author from Lake Tekapo, New Zealand. Credit: Paul Sutherland
Andrew Fox, an ESA-supported member of staff at the Space Telescope Science Institute, in Baltimore, USA, said: “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.
“This inner region is very similar in composition to the Large Magellanic Cloud, suggesting it was ripped out of that galaxy more recently.”
Hubble’s findings have shaken up computer models of the Stream which had predicted that the gas would all be from the Small Magellanic Cloud, which has a weaker gravitational pull than its more massive cousin.
Philipp Richter of the University of Potsdam, Germany, is lead author on one of two papers about the findings. He said: “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. Only Hubble is capable of taking measurements like these.”
The Milky Way has a number of satellite galaxies but only the two Magellanic Clouds still retain most of their gas, due to their greater mass. However the pressure from a halo of hot gas surrounding our own galaxy has pushed some of their gas out into space where the gravitational tug-of-war between the two clouds is thought to have formed the stream.
Hubble’s findings will be useful in helping astronomers learn the processes by which galaxies strip gas from one another and form new stars.
The main image was produced from work by David L. Nidever, et al., NRAO/AUI/NSF and Mellinger, Leiden/Argentine/Bonn Survey, Parkes Observatory, Westerbork Observatory, and Arecibo Observatory.