What makes a galaxy fertile, able to be a veritable star nursery? Why do some continue to make stars, while other seem to become barren? Astronomers, after nearly 20 years of research, say they believe star formation in some galaxies ends because they've been "rained out."
It's not rain or snow, of course, but rather a rain of cool gas that normally helps galaxies create stars by coalescing under the force of gravity.
The phenomenon is referred to as cosmic precipitation, a mechanism in which hot gas produces showers of cool gas that fall into a galaxy.
However, when some of those gas clouds "rain down" into the massive black holes that reside in the center of most galaxy clusters, they are heated into jets with blowtorch-like temperatures that prevent surrounding gas from cooling and forming stars.
"We know that precipitation can slow us down on our way to work," says research leader Mark Voit, a professor of astronomy and physics at Michigan State University. "Now we know it can also slow down star formation in galaxies with huge black holes."
The researchers used NASA's Chandra X-ray Observatory to analyze X-ray emissions from around 200 galaxy clusters to find out how this process of gas precipitation affects the regions around some of the universe's larger black holes.
Vast atmospheres of hot gas surround galaxies in these clusters, gas that normally would cool to form many stars -- but that's not what the astronomers saw in many of the clusters, they reported in the journal Nature.
Instead, they observed evidence of only feeble star production.
"Something is limiting the rate at which galaxies can turn that gas into stars and planets," Voit said. "I think we're finally getting a handle on how this all works."
In some galaxies the precipitation of gas plays a vital role in star production, the researchers found, but in other galaxies this precipitation has been shut off.
It may be that the movement of heat around the center of the cluster, perhaps influenced by a collision with another galaxy cluster, "dried up" the precipitation about the central black hole, they say.
"We can say that a typical weather forecast for the center of a massive galaxy is this: cloudy with a chance of heat from a huge black hole," says study co-author Greg Bryan of Columbia University in New York.
Galaxy clusters vary in size, and can be inhabited by anywhere from 50 to 1,000 galaxies.
Our Milky Way galaxy is a member of a cluster dubbed the Local Group, which has about 50 galaxies.
