The shining of the Universe
In the Milky Way the stars are born less than expected, considering the interstellar gas available. Researchers at MIT and Michigan State University have constructed a theoretical model that describes how galaxy clusters may regulate star formation. In their article, published last week in the journal Nature, describe this innovative model.
Do the galaxies not churn out as many stars as they should? Born each year just a handful of new stars in the Milky Way, and many others shine in the universe. But astronomers have observed that galaxies should generate a much larger number, according to the quantity of interstellar gas at their disposal.
When the intergalactic gas cools, it condenses and collapses to form new stars. Scientists have long thought that something prevented the gas to cool enough to allow the formation of other stars, but what would cause this effect has always remained a mystery. For some clusters of galaxies, the researchers say, the intergalactic gas may simply be too hot, in the hundreds of millions of degrees Celsius. Even if one were to experience a cooling region, the intensity of heat prevented the surrounding region to cool further, an effect known as conduction.
“It would be like putting an ice cube in boiling water. The average temperature would remain very high, “says Michael McDonald, a researcher at the Kavli Institute for Astrophysics and Space Research at MIT. “At these temperatures, the super-conduction flattens the distribution of temperatures, and this prevents the formation of cold clouds, within which may form stars.” To get the so-called clusters of galaxies with “cold nuclei”, the gas in the vicinity of the center must be cold enough to form stars. However, a portion of this cooled gas can slip into the black hole at the center of a galaxy, which then spits out hot material. The material issued in turn heats the surrounding materials, thereby preventing the formation of stars. This effect is called “feedback due to the fall.”
“Some stars are formed, but before the process to take root, the black hole will heat all over again. It’s like the thermostat of storage, “says McDonald. “The combination of conduction and feedback due to the fall provides a simple and clear as star formation functions in galaxy clusters.”
In the universe there are two major classes of galaxy clusters: those with cold core, or those that they cool rapidly and form stars, and those with no cold core, or those who do not have sufficient time to cool. The Coma Cluster, a cluster that is not cold, it’s full of gas that reach temperatures of 100 million degrees Celsius. To be able to form stars this gas should cool for billions of years. In contrast, the cluster of Perseus has the cold core, so the temperatures of its interstellar gas arriving in a few million degrees Celsius. By cooling of the gas present in the cluster of Perseus new stars are born occasionally, though not as many provided by scientists.
“The amount of fuel to power the star formation exceeds ten times the amount of stars observed. These clusters should be a lot richer in stars, “says McDonald. “We need a mechanism that prevents the gas to cool, otherwise the universe should have about ten times more stars than we see.”
McDonald and his colleagues have developed a theoretical model that takes into account two anti-cooling mechanisms. The researchers calculated the behavior of the intergalactic gas basis of average values of radius, mass, density and temperature for a cluster. What we have found is that there is a critical temperature threshold below which the cooling of the gas accelerates significantly, causing a cooling of the gas quickly enough to allow the formation of stars.
According to this theoretical model, there are two different mechanisms that regulate star formation, depending on the storage of located under or above the temperature threshold. For clusters significantly above the threshold, the driving damps the formation of stars: the surrounding gases are too hot because they can survive pockets of cold gas, keeping the material of storage at high temperatures.
“These hot clusters will never form stars, because their temperature will never subside enough,” says McDonald. “Once you enter this regime of high temperatures, cooling is inefficient and clusters remain forever at high temperatures.”
The researchers studied the trend of cooling times of each cluster of galaxies known, comparing it to their theory, and found that the clusters were subdivided into two populations: one of the clusters to slow cooling and those in rapid cooling. These two groups are very close to the threshold value predicted from theoretical study.
Using their model, McDonald claims to predict the evolution of clusters of galaxies and their star formation. “We have built a track on which the clusters are positioned,” says McDonald. “The simple and elegant appearance of this model is that it is anchored to one of two scenarios for a very long time, until something catastrophic happens, like a head-on collision with another cluster.” The researchers hope to be able to better analyze the behavior of this model, to discover whether the mechanism that regulates the star formation in clusters also apply to individual galaxies. The first available data seem to suggest so. “If we can use this information to understand why the stars are formed or not formed around us we would have made a huge step forward in this field,” concludes McDonald.