Hubble Telescope Reveals Source of Gas Streaming Into Milky Way

The Leading Arm of the Magellanic Stream originates primarily from a nearby dwarf galaxy called the Small Magellanic Cloud

**River of Gas**: A photo mosaic of an edge-on view of the Milky Way galaxy, looking toward the central bulge. The arc-shaped Magellanic Stream stretches below the galaxy’s plane and the shredded, fragmented Leading Arm crosses the galaxy’s plane and extends above it. New research reveals that gas in the Leading Arm mostly originates from the Small Magellanic Cloud, seen here as a small bright spot in the middle of the Magellanic Stream. Credit: NASA

The Milky Way is gobbling up more than a billion suns’ worth of gas, and astronomers now have a better idea as to the source of that galactic feast. By combining numerical simulations with observations by NASA’s Hubble Space Telescope, the astronomers identified that the so-called Leading Arm of gas descending into our galaxy mostly originates from a nearby dwarf galaxy called the Small Magellanic Cloud. The researchers reported the discovery on February 21 in The Astrophysical Journal.

The finding offers new insights into how the Milky Way gets its gas, says study co-author Elena D’Onghia of the University of Wisconsin-Madison, who contributed to the work as a visiting scientist at the Flatiron Institute’s Center for Computational Astrophysics (CCA).

“This gas will go on to form new stars in the Milky Way,” D’Onghia says. “Without this gas, the Milky Way will not be able to keep forming stars at the rate we observe today. So, this gas is essentially refurnishing our galaxy.”

The Small Magellanic Cloud and its bigger sibling the Large Magellanic Cloud both closely orbit our galaxy. But as they go around the Milky Way, they also orbit each other. Each one tugs at the other, and one of them has pulled out a massive 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 length 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 collection of gas is being devoured by the Milky Way and, after cooling, will feed new star formation. But scientists wondered which dwarf galaxy is doing the pulling, and whose gas is now being feasted upon.

“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,” explains 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?”

The research is a follow-up to Fox’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 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 process 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,” says collaborator Kat Barger at Texas Christian University.

D’Onghia used numerical simulations to predict how tidal interactions between the Milky Way and the two Magellanic Clouds could strip gas from the dwarf galaxies into their colossal neighbor. Those simulations guided the analysis and interpretation of the team’s observations.

Fox, D’Onghia, Barger and collaborators 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,” explains 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,” 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 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,” explains 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 work 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.

As Fox and his team look ahead, they hope to map out the full size of the Leading Arm—something that is still unknown. When D’Onghia returns to CCA later this year on a sabbatical, she and her team will conduct large-scale numerical simulations to produce more accurate predictions of the amount of gas and chemical composition of the Leading Arm. Those predictions will help reconstruct the history of the puzzling gas feature, she says.

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.