A surprisingly normal galaxy has been discovered in the very early Universe, providing new information on the beginnings of star formation. The galaxy, A1689-zD1, is one of the oldest and most distant ever discovered. It dates to the epoch of re-ionization, a period in the Universe’s history when the first stars formed beginning about 560 million years after the Big Bang. The galaxy itself is visible as it was about 700 million years into the Universe’s history, or about five percent of its 13.8-billion-year lifespan.

The observation was done using the Very Large Array’s X-Shooter instrument along with the Atacama Large Millimeter/submillimeter Array (ALMA), and it relied on gravitational lensing. The galaxy’s image was magnified over nine times by the galaxy cluster Abell 1689 which lies between zD1 and us—the cluster’s gravity acts as a lens, making the galaxy detectable. Without the magnification, it would have been too dim to observe.

“After confirming the galaxy’s distance using the VLT, we realized it had previously been observed with ALMA,” said Darach Watson, the paper's lead author from the Dark Cosmology Center at the University of Copenhagen. "We didn’t expect to find much, but I can tell you we were all quite excited when we realized that not only had ALMA observed it, but that there was a clear detection. One of the main goals of the ALMA Observatory was to find galaxies in the early Universe from their cold gas and dust emissions—and here we had it!”

A1689 is much smaller and less luminous than other galaxies that have been studied in the early Universe, but its discovery is nonetheless important. It likely represents a more typical galaxy of that era than the ones previously discovered. The galaxies that have been discovered have been spotted not because they’re more common, but rather because their large mass makes them easier to detect.

zD1 is surprisingly mature for its age. It has a rich chemical complexity and plenty of dust, both signs of an older galaxy. That’s because the elements heavier than hydrogen and helium—called 'metals’ in astronomical terms—are typically generated in the cores of stars over their lifespans. When the stars die, those metals can be ejected into space. Over time, as many generations of stars die, a significant amount of metals (such as carbon, oxygen, and nitrogen) build up in the galaxy. But zD1 is so young, researchers didn’t expect to find so much metal. Its presence implies that the galaxy had been churning out stars for some time.

This dust was detected because of the galaxy's emission of light in the far-infrared part of the electromagnetic spectrum, in wavelengths characteristic of specific elements. And not only did zD1 have dust, relative to the amount of gas in the galaxy, the dust levels were similar to more mature galaxies. Its dust-to-gas ratio, in other words, is close to that of galaxies like the Milky Way, which have had far more time to produce it.

“Although the exact origin of galactic dust remains obscure, our findings indicate that its production occurs very rapidly, within only 500 million years of the beginning of star formation in the Universe—a very short cosmological time frame, given that most stars live for billions of years,” explained Watson.

The beginning of the Epoch of Re-ionization marked the end of a long era known as the Dark Ages, in which the Universe was either too hot and dense to allow protons and electrons to combine into hydrogen, or the only light present came from what we now view as the cosmic microwave background.

A1689 must have been forming stars at a moderate rate for its entire lifespan to produce the mature galaxy observed. Another possibility is that it went through an early period of starburst—a period of extremely rapid star formation—before dropping off to a more average rate at the time we've observed it.

The researchers were surprised to discover such a galaxy this way, as it was believed that these extremely distant galaxies would be hard to spot. But zD1 was detected with only a brief “glance” by ALMA, increasing the likelihood that more galaxies like zD1 can be detected.

“This amazingly dusty galaxy seems to have been in a rush to make its first generations of stars,” said Kirsten Knudsen of the Chalmers University of Technology, Sweden, one of the paper’s authors. “In the future, ALMA will be able to help us to find more galaxies like this and learn just what makes them so keen to grow up.”

Nature, 2015. DOI: DOI: 10.1038/nature14164 (About DOIs)