Stellar Baby Boom of Early Universe Explained
A typical massive galaxy that existed about 5.5 billion years after the Big Bang. The left image was taken in visible light with the Hubble Space Telescope. The right image is an overlay of the Plateau de Bure millimetre interferometer image.
Credit: MPE/IRAM

The stellar birth rate of the young universe has long been known to be much higher than it is today, but scientists weren't sure why the early universe was so fertile.

A new study finds that this could be because early galaxies had more cold gas to "feed" to forming baby stars.

When astronomers look far across the cosmos with powerful telescopes, they?re effectively looking back in time. Galaxies in the early universe ? a mere 3 or 4 billion years after the Big Bang ? seem to have formed stars more or less continuously with at least ten times the rate seen in similar-mass systems in the local universe, which represents modern time in the 13.7-billion year history of the universe.

The fundamental question facing astronomers was whether these large star-formation rates were caused by larger reservoirs of cold molecular gas (which represents the ?food'? for newly formed stars), or whether star formation in the young universe was much more efficient than it is today.

A major study of distant, luminous star-forming galaxies at the Plateau de Bure millimeter interferometer in the French Alps has now resulted in a breakthrough by getting a direct look at the star-formation "food."

"We have been able, for the first time, to detect and image the cold molecular gas in normal star-forming galaxies, which are representative of the typical massive galaxy populations shortly after the Big Bang," said Linda Tacconi from the Max Planck Institute for Extraterrestrial Physics and lead author of the paper describing the research, which is detailed in the Feb. 11 issue of the journal Nature.

The study showed that galaxies present when the universe was only about 40 percent of its current age had five to 10 times as much gas and more recently formed galaxies do.

The finding fits in with theories on galaxy formation and evolution. As galaxies form and grow, gas cooled and pooled up in concentrations of mysterious dark matter (so-called "dark matter halos"). Over cosmological timescales, gas accreting from these halos onto the proto-galaxies, and collisions and mergers of galaxies, subsequently led to the hierarchical build-up of galaxy mass.

"Given that these galaxies were forming gas at a high rate over long periods of time, this means that gas must have been continuously replenished by accretion from the dark-matter halos, in excellent agreement with recent theoretical work," Tacconi said.

The scientists think these results will help revise future models of the evolution of galaxies.

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