Large galaxies that were present in the first few billion years after the big bang were bloated on gas, resulting in a rash of star formation that outstripped the current rate at which stars are born in the Universe today by up to ten times.

Astronomers have known that stars formed in far higher numbers in the early Universe, and that there was a greater abundance of massive stars in the past, but until now there has never been a satisfactory understanding of why this was the case. Were star factories simply more efficient in the past, or was gas much more plentiful? New observations made with ground-breaking sensitivity at the Plateau de Bure millimetre Interferometer in the French Alps – the most powerful millimetre interferometer in the world, linking six 15m radio telescopes – have mapped both the distribution and motion of molecular hydrogen gas clouds in galaxies as far away as 10.7 billion light years (a redshift of about 2.3). Previously this had only ever been achieved in brilliantly bright quasars (galaxies with extremely active cores); to observe cold gas in normal galaxies at such a distance “has opened the door for an entirely new avenue of studying the evolution of galaxies,” says Pierre Cox of the Institute for Radio Astronomy in the Millimetre Range (IRAM).

The observations didn’t measure the cold hydrogen directly, but instead latched onto the easier-to-see emission lines from carbon monoxide molecules, which are also present within the clouds. Huge reservoirs of gas were detected, confirming that there was more gas in galaxies ten billion years ago than there is today. This gas is accumulated from cannibalisation of nearby dwarf galaxies and large amounts of intergalactic gas pulled in by the gravity of the dark matter haloes around the galaxies. The more gas there is, the more stars are born. Because stars much more massive than our Sun are only born in a ratio of one to several hundred lower mass stars, the greater star formation rate results in larger numbers of massive stars that will explode as supernovae.

“These findings provide us with clues and constraints for theoretical models that we will use to study the early phases of galaxy development in more detail,” says Andreas Burkert of the Excellence Cluster Universe at the Technische Universitaet Muenchen in Garching, Germany. “Eventually these results will help us understand the origin and the development of our Milky Way.”