Hubble Measures Content of the Leading Arm of the Magellanic Stream
Credits:Illustration: D. Nidever et al., NRAO/AUI/NSF and A.
Mellinger, Leiden-Argentine-Bonn (LAB) Survey, Parkes Observatory,
Westerbork Observatory, Arecibo Observatory, and A. Feild (STScI). Science: NASA, ESA, and A. Fox (STScI). Release Image
On the outskirts of our galaxy, a cosmic tug-of-war is unfolding—and only NASA’s Hubble Space Telescope can see who’s winning.
The players are two dwarf galaxies, the Large Magellanic Cloud and
the Small Magellanic Cloud, both of which orbit our own Milky Way
Galaxy. But as they go around the Milky Way, they are also orbiting each
other. Each one tugs at the other, and one of them has pulled out a
huge cloud of gas from its companion.
Called the Leading Arm, this arching collection of gas connects the
Magellanic Clouds to the Milky Way. Roughly half the size of our galaxy,
this structure is thought to be about 1 or 2 billion years old. Its
name comes from the fact that it’s leading the motion of the Magellanic
Clouds.
The enormous concentration of gas is being devoured by the Milky Way
and feeding new star birth in our galaxy. But which dwarf galaxy is
doing the pulling, and whose gas is now being feasted upon? After years
of debate, scientists now have the answer to this “whodunit” mystery.
“There’s been a question: Did the gas come from the Large Magellanic
Cloud or the Small Magellanic Cloud? At first glance, it looks like it
tracks back to the Large Magellanic Cloud,” explained lead researcher
Andrew Fox of the Space Telescope Science Institute in Baltimore,
Maryland. “But we’ve approached that question differently, by asking:
What is the Leading Arm made of? Does it have the composition of the
Large Magellanic Cloud or the composition of the Small Magellanic
Cloud?”
Fox’s research is a follow-up to his 2013 work, which focused on a
trailing feature behind the Large and Small Magellanic Clouds. This gas
in this ribbon-like structure, called the Magellanic Stream, was found
to come from both dwarf galaxies. Now Fox wondered about its
counterpart, the Leading Arm. Unlike the trailing Magellanic Stream,
this tattered and shredded “arm” has already reached the Milky Way and
survived its journey to the galactic disk.
The Leading Arm is a real-time example of gas accretion, the process
of gas falling onto galaxies. This is very difficult to see in galaxies
outside the Milky Way, because they are too far away and too faint. “As
these two galaxies are in our backyard, we essentially have a front-row
seat to view the action,” said collaborator Kat Barger at Texas
Christian University.
In a new kind of forensics, Fox and his team used Hubble’s
ultraviolet vision to chemically analyze the gas in the Leading Arm.
They observed the light from seven quasars, the bright cores of active
galaxies that reside billions of light-years beyond this gas cloud.
Using Hubble’s Cosmic Origins Spectrograph, the scientists measured how
this light filters through the cloud.
In particular, they looked for the absorption of ultraviolet light by
oxygen and sulfur in the cloud. These are good gauges of how many
heavier elements reside in the gas. The team then compared Hubble’s
measurements to hydrogen measurements made by the National Science
Foundation’s Robert C. Byrd Green Bank Telescope at the Green Bank
Observatory in West Virginia, as well as several other radio telescopes.
“With the combination of Hubble and Green Bank Telescope
observations, we can measure the composition and velocity of the gas to
determine which dwarf galaxy is the culprit,” explained Barger.
After much analysis, the team finally had conclusive chemical
“fingerprints” to match the origin of the Leading Arm’s gas. “We’ve
found that the gas matches the Small Magellanic Cloud,” said Fox. “That
indicates the Large Magellanic Cloud is winning the tug-of-war, because
it has pulled so much gas out of its smaller neighbor.”
This answer was possible only because of Hubble’s unique ultraviolet
capability. Because of the filtering effects of Earth’s atmosphere,
ultraviolet light cannot be studied from the ground. “Hubble is the only
game in town,” explained Fox. “All the lines of interest, including
oxygen and sulfur, are in the ultraviolet. So if you work in the optical
and infrared, you can’t see them.”
Gas from the Leading Arm is now crossing the disk of our galaxy. As
it crosses, it interacts with the Milky Way’s own gas, becoming shredded
and fragmented.
This is an important case study of how gas gets into galaxies and
fuels star birth. Astronomers use simulations and try to understand the
inflow of gas in other galaxies. But here, the gas is being caught
red-handed as it moves across the Milky Way’s disk. Sometime in the
future, planets and solar systems in our galaxy may be born out of
material that used to be part of the Small Magellanic Cloud.
The team’s study appears in the Feb. 20 issue of The Astrophysical Journal.
As Fox and his team look ahead, they hope to map out the full size of the Leading Arm—something that is still unknown.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy, in Washington, D.C.
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Contact
Ann Jenkins / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
jenkins@stsci.edu / villard@stsci.edu
Andrew Fox
Space Telescope Science Institute, Baltimore, Maryland
410-338-5083
afox@stsci.edu
Source: HubbleSite/News
