Urban Galactic Clusters Stopped Forming Stars Billions Of Years Ago
December 19, 2013

Urban Galactic Clusters Stopped Forming Stars Billions Of Years Ago

Brett Smith for redOrbit.com - Your Universe Online

A new study based on data from NASA’s Spitzer Space Telescope has revealed new details about so-called urban galaxies that exist in large clusters.

The study found that these clusters stopped forming new stars about 9 billion years ago, much later than previously thought. These galactic cities were said to have eventually either used up or lost their fuel. Meanwhile, more rural galaxies are still actively forming stars.

"We know the cluster galaxies we see around us today are basically dead, but how did they get that way?" asked study author Mark Brodwin of the University of Missouri-Kansas City. "In this study, we addressed this question by observing the last major growth spurt of galaxy clusters, which happened billions of years ago."

In the report, which was published in the Astrophysical Journal, the research team used Spitzer to examine 16 galaxy clusters that formed between 4.3 and 6 billion years after the Big Bang. The researchers were able to essentially look back in time because light from these galaxies took billions of years to reach us. Because Spitzer can see in the infrared spectrum, it was used to detect the dust being warmed by new stars, which the scientists consider a sign of star-formation rates.

Previous observations of closer clusters indicated that the urban galaxies generated all their stars early on in the existence of our nearly 14-billion-year-old universe. These observations support a theory called monolithic collapse, which said these tightly-packed galaxies would have feverishly consumed all their fuel at once. However, the new study indicated that galactic cities continued to produce stars for much longer – until around 9 billion years ago, or around 3 billion years later than previously thought.

In another recent study published in the Monthly Notices of the Royal Astronomical Society, researchers came to a similar conclusion. The study was based on observations from the Herschel Space Observatory of 300 clusters over a longer period of time, when the universe was between 4 and 10 billion years old. Herschel was able to pick up longer wavelengths of infrared light than Spitzer, meaning the two studies essentially complement each other.

"We find that around 9 billion years ago, cluster galaxies were as active as their counterparts outside of clusters; however, their rate of star formation decreases dramatically between then and now, much more quickly than we see in isolated galaxies," said study author Stacey Alberts, an astronomer at the University of Massachusetts-Amherst.

A crowded galactic environment frequently causes galaxies to collide and merge. These mergers result in bouts of fuel-consuming star formation and the growth of supermassive black holes, which eventually blast out gas-heating radiation that shuts off the star formation.

"It's as if boom times for galaxies in clusters ended with a sudden widespread collapse," said Peter Eisenhardt of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "They go from vibrantly forming new stars to the quiescent state they've been in for the last half of the history of the universe, and the switch happens surprisingly fast."