Some galaxies grew up in a hurry. Most of the galaxies that have been observed from the early days of the universe were young and actively forming stars. Now, an international team of astronomers, including Carnegie's Eric Persson and Andy Monson, have discovered galaxies that were already mature and massive in the early days.

Fifteen mature galaxies were found at a record-breaking average distance of 12 billion light years, when the universe was just 1.6 billion years old. Their existence at such an early time raises new questions about what forced them to grow up so quickly. The finding is published by The Astrophysical Journal Letters.

Today the universe is filled with galaxies that have largely stopped forming stars, a sign of galactic maturity. But in the distant past, galaxies were still actively growing by consuming gas and turning it into stars. This means that mature galaxies should have been almost non-existent when the universe was still young.

Together with lead author Caroline Straatman and principal investigator Ivo Labbe, both of Leiden University, the astronomers used deep 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.

The galaxies are barely detectable at visual wavelengths and are easily overlooked. But in the new near-infrared light images they are easily measured, from which it can be inferred that 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, which still forms new stars at a slow rate. The newly discovered galaxies must have formed very rapidly in roughly 1 billion years, with explosive rates of star-formation. The rate of star formation must have been several hundred times larger than observed in the Milky Way today.

The finding raises new questions 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. Another big question is what caused the galaxies to mature at such a young age and if some dramatic event might have caused premature aging.

The galaxies were discovered after 40 nights of observing with the FourStar camera on the Magellan Baade Telescope at Carnegie's Las Campanas Observatory in Chile and combined with data from Hubble's Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and the Great Observatories Origins Deep Survey. Using special filters to produce images that are sensitive to narrow slices of the near-infrared spectrum, the team was able to measure accurate distances to thousands of distant galaxies at a time, providing a 3-D map of the early universe.