By Gail Gallessich

On Earth, thieves steal everything from diamonds to art to money. In space, gas, the fuel for stars, is a commodity worth stealing.

New observations from NASA’s Spitzer Space Telescope reveal a distant, massive galaxy in the act of ripping off vast reservoirs of gas—the equivalent of one billion suns—from a smaller, neighboring galaxy. The stolen gas, which has become scorching hot during the heist, will likely cool and become new stars and planets.

“We may be viewing the larger galaxy in a rare, brief stage of its reincarnation from an old galaxy to a youthful one studded with brilliant stars,” said Patrick Ogle of NASA’s Spitzer Science Center at the California Institute of Technology in Pasadena.

Ogle is the lead author of a new paper on these findings to appear in a recent issue of the Astrophysical Journal. Robert Antonucci, UCSB professor of physics, is one of three co-authors.

The robber galaxy, called 3C326 North, is about the size of the Milky Way galaxy, and its victim, 3C326 South, is about half its mass. They are close enough to perturb each other gravitationally, and might eventually collide. “This could be an important phase in galaxy mergers that we are just now witnessing,” said Ogle.

He and his colleagues initially set out to study a set of distant galaxies, called radio galaxies, about one billion light years away. Radio galaxies are named after the radio-emitting jets that scream out of the black holes at their centers. Though these jets are powerful, their black holes are relatively sleepy and don’t otherwise give off a lot of energy. When the astronomers scanned 72 galaxies using the Spitzer telescope’s infrared vision, they noted a handful that were unusual.

The most extreme of the bunch, 3C 326 North, was soaking in an enormous amount of hot hydrogen gas, reaching temperatures up to 730 degrees Celsius (1,340 degrees Fahrenheit). This gas, called molecular hydrogen gas because it contains molecules of two hydrogen atoms joined together, is a building block of galaxies, stars, and planets.

“Hydrogen is by far the predominant element in the universe, yet in its molecular form it has been virtually undetectable until the Spitzer Space Telescope was launched,” said Antonucci. “The reason is that it is a symmetric molecule, which prevents strong emission or absorption of light at almost any wavelength.

“Molecular hydrogen does produce light in a very specific pattern in the infrared part of the spectrum, and suddenly astronomers are finding huge masses of this key gas, providing big surprises in many fields of astronomical study.”

Since 3C326 North is not making new stars, it is unusual for it to have this much gas. When the researchers investigated the Spitzer pictures further, they noticed what appeared to be a tail of stars, called a tidal tail, connecting 3C326 North to 3C326 South. That made them realize that the pair were interacting, and that 3C326 North was lifting the gas off its companion’s back.

In addition, the hydrogen gas is hot enough to react with oxygen and form vast quantities of water. This water could potentially be incorporated into planets and comets once the gas has cooled enough to collapse and form new stars and planetary systems.

What will happen to the victim galaxy now that it’s bereft of its gas? According to Ogle, the galaxy lost a lot of its fuel and will no longer produce new stars. However, if the two galaxies do eventually merge, then what belongs to one will belong to the other.