Astronomers have discovered a distant galaxy whose stars were born unexpectedly early in our Universe’s history. The observation, which would have been impossible without a cosmic gravitational lens, sheds new light on the formation of the first galaxies as well as on the evolution of the early Universe.

Clusters of galaxies are the largest gravitationally bound structures in our Universe. Their gravity is so strong that they bend the ray of lights coming from galaxies located behind them. As a result, multiple and often distorted images of the background galaxy are produced (see the animation below). These gravitational lenses also amplify the light coming from these otherwise invisible objects.

© NASA, ESA & L. Calçada


Johan Richard and his colleagues used the cluster named Abell 383 to identify a galaxy so far away that we see it as it was when the Universe was around 950 million years old. The team of researchers used recent observations from the NASA/ESA Hubble Space Telescope, and then verified everything with observations from the NASA Spitzer Space Telescope. They finally measured the galaxy’s distance using the Keck-II telescope in Hawaii. The research will appear in a paper entitled “Discovery of a possibly old galaxy at z=6.027, multiply imaged by the massive cluster Abell 383”, to be published in the Monthly Notices of the Royal Astronomical Society.


© NASA, ESA, J. Richard (CRAL) and J.-P. Kneib (LAM). Acknowledgement: Marc Postman (STScI)


Actually, more distant galaxies have already been observed: one of these galaxies, for example, is seen as it was when the Universe was around 450 million years old. However, the newly discovered galaxy does not shine brightly with only young stars, unlike other remote galaxies. Spitzer revealed that the galaxy is made of very old and relatively faint stars, some of them being nearly 750 million years old. In other words, this means the galaxy was formed only 200 million years after the Big Bang: this suggests that galaxies have been around for a lot longer than previously thought.

The discovery may also help explain why the Universe became transparent to ultraviolet light in the first billion years after the Big Bang. In the early Universe, ultraviolet light was blocked by a diffuse fog of neutral hydrogen gas; to make this fog transparent to ultraviolet light as it is today, some source of radiation must have ionized the gas: this process is known as reionization.

It is believed that this source of radiation must have come from galaxies, but their very small number is not enough to provide the necessary radiation. If many other galaxies are as old and faint as the one newly discovered, this could provide the missing radiation that cleared the hydrogen fog.

Unfortunately, for the time being, such galaxies can only be observed through gravitational lenses. The NASA/ESA/CSA James Webb Space Telescope, to be launched in a few years, will be the perfect tool to help solve this mystery.