14 April 2011

First galaxies created earlier than previously thought

A team of astronomers, including members from the Niels Bohr Institute, have discovered a distant galaxy whose stars were created earlier in the history of the universe than previously observed. It sheds new light on the formation of the first galaxies and the evolution of the universe. The results have been published in the Monthly Notices of the Royal Astronomical Society.

"The galaxy we have discovered began to form stars a mere 200 million years after the Big Bang. This is much earlier than previously thought and may change the theories about the evolution of the early universe”, explains PhD in astrophysics Johan Richard, who splits his time between the Centre de Recherche Astronomique de Lyon Université in France and the Dark Cosmology Centre, Niels Bohr Institute at the University of Copenhagen.

Cosmic magnifying glass

Observation of the galaxy

The galaxy was observed with the Hubble Space Telescope and NASA's Spitzer Space Telescope. The observations are very detailed, because the galaxy is visible through a cluster of galaxies, called Abell 383. It is this lucky situation that this galaxy cluster lies between Earth and the galaxy and amplifies the light and makes it possible to observe the galaxy, which otherwise would have been too faint to see.

The galaxy was first observed using the Hubble Space Telescope and then with NASA's Spitzer Space Telescope. The galaxy is visible through a cluster of thousands of galaxies called Abell 383, which has such a strong gravitational pull that the light that the galaxies behind it emit is deflected and focused. The phenomenon is called gravitational lensing and acts like a magnifying glass. This galaxy cluster strongly amplifies the light and makes it possible to observe the galaxy, which would have otherwise been too faint to see.

After having studied the images from the Hubble and Spitzer telescopes, the researchers conducted spectroscopic observations of the galaxy using the Keck-II telescope in Hawaii. Spectroscopy is a technique where light is split up into the colours of the spectrum and by analysing the light from the galaxy the astronomers could measure the distance to the galaxy and infer information about the properties of the stars.

Early star formation

The measurements of the light showed that when the galaxy emitted the light that we are currently seeing the universe was about 950 million years old. That does not mean that this is the most distant galaxy ever observed. A galaxy has been observed, which emitted the light we see now 400 million years ago. But the big difference is the age of the galaxy’s stars.

"By analysing the light wavelengths we can ascertain whether the galaxy is comprised of young or old stars. Young stars are very bright and emit a lot of UV light, while the light of old stars is mostly on the visible light spectrum. What we saw was that the galaxy mainly comprised of old stars. When there are many old stars it means that it began to form its stars much earlier”, explains Johan Richard.

Old stars in distant galaxies mean that the first galaxies have existed for a long time. From the properties and age of the stars the researchers have calculated that the galaxy was already beginning to form stars when the universe was around 200 million years old.

The mystery of the transparency of the universe

The discovery of the galaxy could also help to explain how the universe became transparent to ultraviolet light in the first billion years after the Big Bang. In the early universe a cloud of hydrogen blocked all ultraviolet light. A form of radiation may have ionized the hydrogen gas, so that it became transparent for UV light, as the universe is today. This radiation must have come from the galaxies, so the discovery of this early galaxy may be the answer to this mystery, because there may have been far more galaxies in the early universe than previously estimated.

Currently, we can only detect the very early, distant galaxies by being so incredibly lucky to observe them through a large collection of galaxies, which can function as a cosmic magnifying glass. But with the new James Webb Space Telescope, which will be launched in a few years, the astronomers will be able to observe the distant galaxies in high resolution and perhaps the mystery of the early universe will finally be solved.