We know that once galaxies grow older, their star-making abilities cease. But what causes this halt in star formation?
For NGC 3226, a galaxy 50 million light-years away from Earth, scientists discovered that the answer lies with a surge of warm gas that came into the galaxy as a result of it colliding with another galaxy.
There are two major kinds of galaxies in the universe. The first is like our Milky Way, younger star-forming galaxies. Then there are older galaxies, ones where star formation no longer occurs. However, NGC 3226 lies somewhere between the two, a middle-aged galaxy. But it's not producing new stars. Why?
Stars form in galaxies where gas is cool and calm. However, recently, NGC 3226 collided with a neighboring galaxy, which caused lots of warm gas to rush in. This disturbed the cool gases within the galaxy and put a halt to NGC 3226's star-making abilities.
"We have explored the fantastic potential of big data archives from NASA's Hubble, Spitzer and ESA's Herschel observatory to pull together a picture of an elliptical galaxy that has undergone huge changes in its recent past due to violent collisions with its neighbors," says Philip Appleton, project scientist for the NASA Herschel Science Center. "These collisions are modifying not only its structure and color, but also the condition of the gas that resides in it, making it hard -- at the moment -- for the galaxy to form many stars."
But how do we know this? Observations from all three telescopes show "streamers of material" throughout NGC 3226, suggesting the existence of a galaxy that NGC 3226 basically devoured. One of these filaments resembles a tail that stretches into NGC 3226's core, pulled there by gravity.
Normally, when this sort of material ends up in a galaxy, dust and gas start forming stars. But in this case, the material is too hot for star formation.
These observations are important because they help us understand what happens when a galaxy evolves over time. We know that NGC 3226 is middle-aged because of the colors of its stars. Newer stars show up bright blue in images, but older stars cast a reddish light. NGC 3226 has both, suggesting that its age is neither young or old, but somewhere in the middle.
Because of these new observations, we see how these middle-aged galaxies can lose their star formation abilities. However, astronomers believe that this is only temporary for NGC 3226, as it will eventually cool back down, providing a more appropriate temperature for star-making.
"NGC 3226 will continue to evolve and may hatch abundant new stars in the future," says Appleton. "We're learning that the transition from young- to old-looking galaxies is not a one-way, but a two-way street."
[Photo Credit: NASA/CFHT/NRAO/JPL-Caltech/Duc/Cuillandre)