In a cosmic tug-of-war between Large and Small Magellanic Clouds — two dwarf galaxies orbiting the Milky Way — only the NASA/ESA Hubble Space Telescope can see who’s winning. As they gravitationally tug at each other, one of these dwarf galaxies has pulled out a huge amount of gas from its companion. Called the Leading Arm, this 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? Space Telescope Science Institute astronomer Andrew Fox and co-authors used Hubble’s ultraviolet (UV) vision to chemically analyze the gas in the Leading Arm and determine its origin.
Large and Small Magellanic Clouds gravitationally tug at each other, and one of them has pulled out a huge amount of gas from its companion; this shredded and fragmented gas, called the Leading Arm, is being devoured by the Milky Way and is feeding new star birth in our Galaxy. Using Hubble data, Fox et al have now solved which dwarf galaxy is doing the pulling. Image credit: Nidever et al / NRAO / AUI / NSF / Mellinger / Leiden-Argentine-Bonn / LAB Survey / Parkes Observatory / Westerbork Observatory / Arecibo Observatory / Field / STScI / NASA / ESA / A. Fox.
Roughly half the size of the Milky Way, the Leading Arm 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.
“There’s been a question: did the gas come from the Large Magellanic Cloud or the Small Magellanic Cloud?” Dr. Fox said.
“At first glance, it looks like it tracks back to the Large Magellanic Cloud. 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?”
This study is a follow-up to the team’s 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 the astronomers 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,” they said.
“This is very difficult to see in galaxies outside the Milky Way, because they are too far away and too faint.”
“As Large and Small Magellanic Clouds are in our backyard, we essentially have a front-row seat to view the action,” said Dr. Kat Barger, from Texas Christian University.
In a new kind of forensics, the study authors used Hubble’s UV 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, they measured how this light filters through the cloud.
In particular, they looked for the absorption of UV 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 NSF’s Robert C. Byrd Green Bank Telescope at the Green Bank Observatory in West Virginia, as well as several other radio telescopes.
After much analysis, the scientists 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. That indicates the Large Magellanic Cloud is winning the tug-of-war, because it has pulled so much gas out of its smaller neighbor,” Dr. Fox said.
The findings are published in the Astrophysical Journal.
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Andrew J. Fox et al. 2018. Chemical Abundances in the Leading Arm of the Magellanic Stream. ApJ 854, 142; doi: 10.3847/1538-4357/aaa9bb
