Sen—Astronomers using one of the world’s most powerful telescopes have captured images of some of the first galaxies to shine as the Universe switched on.

And they believe they appeared quite suddenly in the early Universe, less than 700 million years after the Big Bang.

This explosive start to the Universe, 13.8 billion years ago, filled space with a hot “soup” of charged protons and electrons. It was followed by what cosmologists call the Dark Ages because everything cooled to become an opaque fog of neutral hydrogen atoms.

Eventually however, the first stars and galaxies began to form as these atoms were excited, or ionised, and began to glow. The fog finally cleared completely about 12.8 billion years ago, a billion years after the Big Bang.

When you look deep into the Universe, you look back in time. And astronomers have long been looking for examples of the first galaxies to form in the furthest reaches of space. Now a team using Japan's Subaru Telescope at Mauna Kea on Hawaii have found seven of them.

The astronomers, led by student Akira Konno and Dr Masami Ouchi of the University of Tokyo, were seeking a particular type of galaxy called a Lyman-alpha emitter (LAE) to see what role it might have played in cosmic reionisation.

They are so named because they are lit up by powerful hydrogen excitation, known as Lyman-alpha emission.

The seven LAE galaxies found by the team as they appeared 13.1 billion years ago. Marked by the white dashes, the galaxies appear reddened by the effects of cosmic expansion. Image credit: ICRR,/University of Tokyo/NAOJ

The discovery team used a camera called the Suprime-Cam on Subaru to perform the Subaru Ultra-Deep Survey for Lyman-alpha Emitters. And they had to expose the camera for a total of nearly four and a half days to collect enough light.

Konno said in a statement: “It is quite difficult to find the most distant galaxies due to the faintness of the galaxies. So, we developed a special filter to be able to find a lot of faint LAEs. We loaded the filter on to Suprime-Cam and conducted the most distant LAE survey with the integration time of 106 hours.”

This length of time was one of the longest ever carried out with Subaru and allowed it to find the most distant galaxies it has every recorded, at a distance of 13.1 light-years. More distant galaxies have been found, however, by the Hubble Space Telescope.

What came as a suprise to the astronomers was the small number of LAE galaxies that they detected. They expected to mop up several tens of them rather than the seven they captured. But then they realised that this was telling them how swiftly the galaxies switched on and began to glow.

Konno said: “At first we were very disappointed at this small number. But we realized that this indicates LAEs appeared suddenly about 13 billion years ago. This is an exciting discovery.”

The team believe their observations show that the fog that kept the early Universe dark lifted around 13 billion years ago so that the LAE galaxies suddenly came into view for the first time. But they are investigating other possibilities too, such as the disappearance of clumps of neutral hydrogen surrounding them, or that the LAEs became signigicantly brighter themselves.