The Magellanic Stream
Credit for the radio/visible-light image:
David L. Nidever et al., NRAO/AUI/NSF and A. Mellinger, LAB Survey,
Parkes Observatory, Westerbork Observatory, and Arecibo Observatory. Credit for the radio image: LAB Surve. More Images
Astronomers using NASA's Hubble Space Telescope have solved a 40-year
mystery on the origin of the Magellanic Stream, a long ribbon of gas
stretching nearly halfway around our Milky Way galaxy.
The Large and Small Magellanic Clouds, two dwarf galaxies orbiting
the Milky Way, are at the head of the gaseous stream. Since the
stream's discovery by radio telescopes in the early 1970s, astronomers
have wondered whether the gas comes from one or both of the satellite
galaxies. Now, new Hubble observations reveal that most of the gas was
stripped from the Small Magellanic Cloud about 2 billion years ago, and a
second region of the stream originated more recently from the Large
Magellanic Cloud.
A team of astronomers, led by Andrew J. Fox of the Space Telescope
Science Institute in Baltimore, Md., and the European Space Agency,
determined the source of the gas filament by using Hubble's Cosmic
Origins Spectrograph (COS) to measure the amount of heavy elements,
such as oxygen and sulfur, at six locations along the Magellanic
Stream. COS observed faraway quasars whose emitted light passes through
the stream and detected these elements from the way they absorb
ultraviolet light. Quasars are the brilliant cores of active galaxies.
Fox's team found a low amount of oxygen and sulfur along most of the
stream, matching the levels in the Small Magellanic Cloud about 2
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. "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," said Fox. "This inner region is very similar in composition to the
Large Magellanic Cloud, suggesting it was ripped out of that galaxy
more recently."
This discovery was a wrinkle Fox's team didn't expect, because
computer models of the stream predicted that the gas came entirely out
of the Small Magellanic Cloud, which has less gravity than its more
massive cousin.
"Only Hubble can measure these abundances," Fox explained. "You have
to go to space because the absorption lines we need to measure these
abundances are all in the ultraviolet, and Earth's atmosphere absorbs
ultraviolet light."
Astronomers have debated whether the two Magellanic Clouds are on their first pass near our Milky Way or are bound to it.
"What's interesting is that all the other nearby satellite galaxies
of the Milky Way have lost their gas," Fox said. "The Magellanic Clouds
have been able to retain their gas and are still forming stars because
they're more massive than the other satellites. However, as they're
now approaching the Milky Way, they're feeling its gravity more and
also encountering its halo of hot gas, which puts pressure on them.
That process, together with the gravitational tug-of-war between the
Magellanic Clouds, leads to the production of the stream. You're seeing
material stripped out of the Clouds as they come in toward the Milky
Way."
Ultimately, the gaseous stream may rain down onto the Milky Way's
disk, fueling the birth of new stars. This infusion of fresh gas is
part of one process that triggers star formation in a galaxy.
Astronomers want to know the origin of that wayward gas in order to
more fully understand how galaxies make new stars.
"We want to understand how galaxies like the Milky Way strip the gas
from small galaxies that fall into them and use that to form new
stars," Fox explained. "This seems like it's an episodic process. It's
not a smooth process where a slow stream of gas comes in continuously.
Instead, once in a while a large gas cloud falls in. We've got a way of
testing that here, where two galaxies are coming in. We have shown
which of them is producing the gas that ultimately will fall into the
Milky Way."
The team reported its results in two papers that appeared in the Aug.
1 issue of The Astrophysical Journal. Fox is the lead author of one
paper; the other paper's lead author is Philipp Richter of the
University of Potsdam in Germany.
CONTACT
Donna Weaver / Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4493/4514
dweaver@stsci.edu / villard@stsci.edu
Andrew Fox
Space Telescope Science Institute, Baltimore, Md.
410-338-5083
afox@stsci.edu
