Some galaxies in early universe grew up quickly

A team of researchers has discovered the most distant examples of galaxies that were already mature and massive.

The mature galaxies were found at a record-breaking distance of 12 billion light-years, seen when the universe was just 1.6 billion years old.

Graduate student and first author Caroline Straatman (Leiden University, The Netherlands), said that today the universe is old and filled with galaxies that have largely stopped forming stars, a sign of galactic maturity.

Astronomers used sensitive images at near infrared wavelengths to search for galaxies in the early universe with red colors. The characteristic red colors indicate the presence of old stars and a lack of active star formation. Surprisingly, they located 15 galaxies at an average distance of 12 billion light-years, only 1.6 billion years after the Big Bang.

The galaxies are barely detectable at visual wavelengths and are easily overlooked, but in the new near infrared light images they are easily seen. The team estimates they already contained as many as 100 billion stars on average per galaxy.

The mature galaxies have masses similar to that of the Milky Way, but were already retired from star-formation when the universe was only 12 per cent of its current age. While the Milky Way currently forms new stars at a slow rate, these galaxies must have formed very rapidly in a very "short time" (roughly 1 billion years), with explosive events in which new stars were being formed. This must have happened at rates several hundred times higher than in the Milky Way today.

"This finding raises new questions," says team leader Dr. Ivo Labbe (Leiden University). "For example about how these galaxies formed so rapidly and why they stopped forming stars so early. It is an enigma that these galaxies seem to come out of nowhere. Large numbers of galaxies with explosive rates of star formation have not yet been identified in the early universe.

The finding has been published in The Astrophysical Journal Letters.