Generating stars

Print edition : April 17, 2015

This Hubble Space Telescope image of the galaxy NGC1275 shows the fine, thread-like filamentary structures surrounding it. The red filaments are composed of cold gas being suspended by a magnetic field and are surrounded by the 100-million-degree gas in the cntre of the Perseus galaxy cluster.

STARS form when intergalactic gas cools rapidly and condenses, which then collapses to form new stars. But, based on the amount of interstellar gas available, galaxies should be churning out millions more stars than they do. For scientists, it has remained a mystery as to what prevents the gas from cooling enough to generate more stars.

Astronomers from the Massachusetts Institute of Technology (MIT) and Michigan State University have proposed a theory describing how clusters of galaxies may regulate star formation. The work has been reported in a recent issue of the journal Nature. “The amount of fuel for star formation outpaces the amount of stars 10 times,” says Michael McDonald, a Hubble Fellow in MIT’s Kavli Institute for Astrophysics and Space Research. “You really need some mechanism to prevent gas from cooling, otherwise the universe would have 10 times as many stars.”

Throughout the universe, there exist two main classes of galaxy clusters: cool core clusters—those that are rapidly cooling and forming stars—and non-cool core clusters —those that do not have sufficient time to cool.

According to the researchers, for some galaxy clusters, the intracluster gas may simply be too hot—of the order of hundreds of millions of degrees Celsius. Even if one region experiences some cooling, the intensity of the surrounding heat would keep that region from cooling further. “It would be like putting an ice cube in a pot of boiling water; the average temperature is pretty much still boiling,” says McDonald. “At super-high temperatures, conduction smooths out the temperature distribution, so you don’t get any of these cold clouds that should form stars.” The group calculated the behaviour of intracluster gas based on a galaxy cluster’s radius, mass, density and temperature. The researchers found that there is a critical temperature threshold below which the cooling of gas accelerates significantly, causing gas to cool rapidly enough to form stars.

According to the new theory, two different mechanisms regulate star formation, depending on whether a galaxy cluster is above or below the temperature threshold. For clusters that are significantly above the threshold, conduction puts a damper on star formation.