Why fewer stars are born today than in the early universe

University of Arizona (UA) astronomers have helped solve why fewer stars are born today than in the early universe, a mystery that has long puzzled scientists.

"We have known for more than a decade that in the early universe - three to five billion years after the Big Bang or nine to eleven billion years before today - galaxies churned out new stars at a much faster rate than they do now," said Michael Cooper, a postdoctoral Spitzer fellow at the UA's Steward Observatory.

"What we haven't known is whether this was because they somehow formed stars more efficiently or because more raw material - molecular gas and dust - was available," said his colleague Benjamin Weiner, an assistant astronomer at Steward Observatory and one of the co-authors on the paper.

Compared to the average galaxy today, which produces stars at rates equaling about 10 times the mass of our Sun per year, the rate of star formation in those same galaxies appears to have been up to 10 times higher when they were younger.

In its efforts to find an answer, the scientific community has tended to turn telescopes toward few, rare, very bright objects.

By focusing on the rare, bright objects, the results obtained cast doubts as to whether they are true for the majority of galaxies populating the universe.

He and his coworkers took advantage of more sensitive instruments and refined surveying methods to hone in on more than a dozen 'normal' galaxies.

"Our study is the first to look at the 'five-foot eight' kinds of galaxies, if you will," Copper said.

"Our results therefore are more representative of the typical galaxy out there. For the first time, we are getting a much more complete picture of how galaxies make stars," he added.

Cooper and his colleagues used data from an earlier study, in which they had surveyed about 50,000 galaxies, to pick a sample representing an 'average' population of galaxies.

They then pointed various telescopes toward their study objects.

"By observing those galaxies in the infrared spectrum and measuring their radio frequency emissions, we were able to make their cold gas clouds visible," explained Cooper.

"What we found now is that galaxies like the ancestors of the Milky Way had a much greater supply of gas than the Milky Way does today," said Weiner.

"Thus, they have been making stars according to the same laws of physics, but more of them in a given time because they had a greater supply of material," he added.