Just a few stars each year are born in the Milky Way, but astronomers' observations of interstellar gas suggest that galaxies should be pumping out more.
So, why isn't the universe brighter?
Researchers from MIT and Michigan State University have a theory that would explain how galaxy clusters might regulate star formation, according to a press release from MIT. Their theory is explained in the journal Nature.
When gas belonging to a cluster rapidly cools, it condenses. That causes the gas to collapse and generate a new star. Initially, scientists thought that something was stopping the gas to cool enough to create new stars - but that "something" was a mystery.
If intracluster gas is too hot (hundreds of millions of degrees Celsius), stars won't form in that cluster, nor will neighboring clusters have a chance. The heat from the hot cluster would keep the entire area warmer due to conduction.
"It would be like putting an ice cube in a boiling pot of water - the average temperature is pretty much still boiling," said Michael McDonald, a Hubble Fellow in MIT's Kavli Institute for Astrophysics and Space Research, according to the press release. "At super-high temperatures, conduction smoothes out the temperature distribution so you don't get any of these cold clouds that should form stars."
In "cool core" galaxy clusters, the central gas might be cool enough for star formation, but if some of the cooled gas trickles or rains into a black hole - which would then shoot out hot material - the surrounding area would again be too warm. That effect is called "precipitation-driven feedback," according to the press release.
"Some stars will form, but before it gets too out of hand, the black hole will heat everything back up - it's like a thermostat for the cluster," McDonald said, according to the press release. "The combination of conduction and precipitation-driven feedback provides a simple, clear picture of how star formation is governed in galaxy clusters."
Cluster Classes
The universe holds two main classes of galaxy clusters: cool core clusters and non-cool core clusters. The latter has not has enough time to cool.
The Coma cluster is an example of a non-cool cluster. The gas in the cluster is 100 million degrees Celsius, so in order for stars to form, the galaxy would need a cooling off period of several billion years. Compare that to the Perseus cluster with intracluster gas at only a few million degrees Celsius. New stars are created by the cooling gasses in Perseus, but astronomers would have predicted more than what the cluster yields.
"The amount of fuel for star formation outpaces the amount of stars 10 times, so these clusters should be really star-rich," McDonald said, according to the press release. "You really need some mechanism to prevent gas from cooling; otherwise the universe would have 10 times as many stars."
Based on the behavior of intracluster gas caused by the cluster's radius, mass, density and temperature, McDonald and his peers worked out a theoretical framework relying on two anti-cooling mechanisms, according to the press release. The theory asserts that two different mechanisms regulate the formation of stars, based on the temperature of the cluster. In clusters that are above the threshold, conduction hinders star birth.
"For these hotter clusters, they're stuck in this hot state, and will never cool and form stars," McDonald said, according to the press release. "Once you get into this very high-temperature regime, cooling is really inefficient, and they're stuck there forever."
For cooler clusters closer to the lower threshold, precipitation-driven feedback puts the kibosh on star creation.
"In the Perseus cluster, we see these jets acting on hot gas, with all these bubbles and ripples and shockwaves," McDonald said, according to the press release. "Now we have a good sense of what triggered those jets, which was precipitating gas falling onto the black hole."
Observations Match the Theory
McDonald and his colleagues found that their observations matched the theory they set forth. Data from the Chandra X-ray Observatory and the South Pole Telescope (an observatory in Antarctica looking for distant massive galaxy clusters) also supports the theory. McDonald said that using the theoretical framework could help astronomers predict galaxy cluster evolution.
"We've built a track that clusters follow," McDonald said, according to the press release. "The nice, simple thing about this framework is that you're stuck in one of two modes, for a very long time, until something very catastrophic bumps you out, like a head-on collision with another cluster."
"If we can use all this information to understand why or why not stars form around us, then we've made a big step forward," McDonald said.
This research was funded in part by the National Science Foundation and NASA.
