Astronomers using NASA’s Hubble Space Telescope have uncovered a distant cluster of galaxies in the initial stages of construction.

The developing proto-cluster, located 13.1 billion light years away, consists of five tiny galaxies – all very young at just 600 million years old. Astronomers imaged the cluster during its initial stages of development, and it is seen as it appeared some 13 billion years ago. As the cluster is observed at such an early stage in its evolution, it is likely that if viewed today it would be significantly more impressive, comparable to “galactic cities” such as our neighbouring Virgo cluster of nearly 2,000 galaxies.

The observations were part of the ‘Brightest of Reionizing Galaxies’ survey, run using Hubble’s wide field camera WFC3. The team conducting the study deliberately hunted for very bright galaxies, as this signifies a cluster construction zone; the team expects many fainter galaxies to reside in the same area as the five observed ones. These bright galaxies lie within wells of invisible dark matter, a mysterious substance that is thought to bind galaxies together like glue.

The newly-observed galaxies are amongst the brightest seen at that epoch, when the earliest galaxies burned off primordial clouds of cold hydrogen in a process known as reionization. Author of the study Michele Trenti of the University of Colorado at Boulder and the Institute of Astronomy at the University of Cambridge said these young galaxies well represent the beginnings of cluster formation in the early Universe. “The result confirms our theoretical understanding of the build-up of galaxy clusters. And, Hubble is just powerful enough to find the first examples of them at this distance.”

The five galaxies are approximately one-half to one-tenth the size of our Milky Way, but are comparable in brightness, showing that they must be very massive. The research team ran computer simulations to show that the galaxies are continually fed by gas, dust and other matter from mergers with neighbouring galaxies. Through time, the team expect the five galaxies to eventually merge to form one incredibly bright large elliptical galaxy at the centre of the cluster.

The new observations will help astronomers understand the process of cluster evolution from the very beginning of their formation. Following the hierarchical model of galactic assembly, small objects and galaxies gain mass – and therefore brightness – by merging with other galaxies in a smooth, steady and continuous growth. These collisions result in large galaxies. The majority of galaxies throughout the Universe are part of a cluster or group, but such early clusters are rare to find because they are dim, widely-scattered, and located at huge distances from us. “The search is hit and miss,” says Trenti. “Typically, a region has nothing, but if we hit the right spot, we can find multiple galaxies.”

The next step for the team is to use spectroscopy to more accurately determine the distance to the observed galaxies, which had been previously calculated using their colours. This will help to reveal the galactic velocities, show the gravitational expansion of the space surrounding the cluster, and show whether the galaxies are gravitationally bound together.

The results were presented earlier this week at the American Astronomical Society meeting in Austin, Texas. The study will be published in an upcoming issue of The Astrophysical Journal.