The mystery of why galaxies formed early in the history of the universe give birth to more stars than modern ones has been solved. An abundance of dense, cold gas fueled rapid star formation in these early galaxies, according to a new study.

Astronomers collected signals from 19 different 8- to 10-billion-year old galaxies scattered across the northern sky. These early-universe stellar nurseries had much more interstellar gas — dense, hydrogen-rich clouds at a chilly minus 441 to minus 414 degrees Fahrenheit — than their modern counterparts.

“This is really pioneering work,”  said astrophysicist Kai Noeske of the Harvard Smithsonian Center for Astrophysics. ”It unambiguously confirms that these galaxies really are more gas-rich, so the reason they made more stars back in the day is that they had more fuel to burn.”

Scientists study distant galaxies because the light they sent out billions of years ago is only now reaching us, and can therefore tell us about conditions early in the universe’s 13.7-billion-year history.

No one knew why stars form more than 10 to 100 times more often in distant, massive galaxies than they do in local galaxies of the same mass, said astronomer Linda Tacconi of the Max Planck Institute for Extraterrestrial Physics in Germany, lead author of the Feb. 10 Nature study.

Some scientists had guessed that these early galaxies contained more cold interstellar gas, which fueled the frenetic birth of stars.  Others argued that these ancient galaxies had the same amount of gas as the Milky Way, but that suns formed in short, furious starbursts as these galaxies collided, Noeske said.

Determining which theory was right was difficult. The cold, dense gas clouds emit such faint, low-energy light that even the most sensitive instruments can barely detect them. Just a few years ago, Tacconi’s team searched for signals from these galaxies, but failed, she said.

The group was finally able to answer the question by adding more-sensitive detectors to the IRAM Plateau de Bure Interferometer, an array of millimeter-wavelength radio telescopes located at 7,381 feet in the French Alps.

Ultimately, the team wanted to know how much hydrogen filled these early galaxies, because it is by far the most abundant element in the universe and in interstellar gas clouds. But hydrogen emissions from these distant objects are simply too hard to detect, Tacconi said.

Instead, they measured the light emitted from carbon monoxide molecules. As these molecules rotate, they shift from one energy state to another. As they shift, “they emit photons, and that radiation is what we see as an emission line at a specific wavelength,” Tacconi said.

The amount of light emitted from these spinning molecules revealed the fraction of each galaxy made up of carbon monoxide. Carbon monoxide and hydrogen are found in almost the same ratio in many parts of the universe. So, they used this ratio to extrapolate the amount of hydrogen present in these early galaxies.

A 10-billion-year-old galaxy was made of about 44 percent cold interstellar gas by mass, while an 8-billion-year-old one was about 34 percent. This is three to 10 times more hydrogen than today’s giant galaxies.

The study also showed the old galaxies drew in fuel from their surrounding environment in order to keep up the frantic pace of star formation, Noeske said.

Future research should look at a larger number of galaxies and find a way to measure smaller galaxies, said astronomer Dawn Erb of the University of California, Santa Barbara, who was not involved in the study.

“This is just the tail end of the population of the normal galaxies, just the biggest and most massive ones,” she said. “We just can’t see the normal ones, because they’re too faint.”

To do that, the team will need even-more-sensitive equipment, which they will get when the ALMA observatory in Chile comes online in 2012. “That’s going to be the next big step,” Erb said.