Astronomers from the University of Tokyo and the National Astronomical Observatory of Japan (NAOJ) have uncovered star-forming galaxies that appear paradoxically red in a galaxy cluster situated some four billion light years away.

Using the Subaru Telescope at the summit of Mauna Kea, Hawaii, the team yielded results that indicated such flame-red galaxies are a transitional phase from a young generation of stars into a much older one, possibly demonstrating the evolution of galaxies in the environment surrounding a galaxy cluster. “Ordinarily, when the overall colour of a galaxy is red, the majority of its population is red coloured stars – namely older stars,” says Saeko Hayashi of the Public Information and Outreach Office at the NAOJ.

Galaxy cluster formation occurred some ten billion years ago, with the galaxies congregating under their own gravity. During their assembly, galactic properties changed in relation to their environments – elliptical and lenticular galaxies are typically found in clusters, while spirals prefer their own company, underlining the fact that the formation and evolution of galaxies is still full of mysteries.

In a bid to answer the burning question of how galactic formation patterns become established and evolve, the research team led by Dr Yusei Koyama used the Subaru Prime Focus Camera to make a panoramic observation of the rich galaxy cluster CL0939+4713, located four billion light years away from Earth. Carefully comparing images taken with and without a filter that can detect the hydrogen-alpha line emitted by hydrogen atoms ionised by ultraviolet light from newborn stars, the team identified more than 400 galaxies in the cluster that illustrated an excess of hydrogen-alpha when observed with a filter. Surprisingly, they discovered that a large number of these star-forming galaxies burned red. “Strong hydrogen-alpha emission means strong star formation and a red colour implies older stars,” says Hayashi. “These two findings do not go together. In the middle of the cluster of galaxies, there are many galaxies with ongoing active star formation. Researchers thought that there is not much going on in the outskirts of the cluster, therefore it was a surprise for Koyama and his team to have found these red-burning galaxies in the periphery of the galaxy cluster.”

While questions regarding the physical origin and distribution of the galaxies were raised, the team are still unsure of the answers. It seems that, at the very minimum, the strong hydrogen-alpha emissions illustrate that the red galaxies are forming new stars. “The red colour could be due to the abundance of dust, rather than by old stellar populations, ” says Hayashi. The researchers believe that strong gravity will play its part in attracting groups of the red galaxies to CL0939+4713 causing them to merge, thus illustrating that the properties of galaxies can change in sparse environments before they coalesce in a rich cluster.

Additionally, Koyama and his team realised that the number of old galaxies, without active star formation, appeared to be increasing in the group environments, precisely where the red galaxies are plentiful. “The galaxies in the outskirts of the cluster have both the red-burning, star forming galaxies and the old galaxies,” says Hayashi. “We think that the red-burning galaxies soon cease active star formation and become members of the older galaxies. The galaxies were caught in a transient phase – a mixture of two different populations to this single constituent of matured galaxies.” The fact that transitional galaxies are found in groups highlights that these galactic gatherings are key in understanding how the evolution of galaxies takes shape.

The results, which are published in The Astrophysical Journal , have been made possible by the wide-field capability of the Subaru Telescope and marks the start of investigating these fire-red galaxies.