Looking back in time to when the Universe was in its formative youth, the Advanced Camera for Surveys (ACS) aboard the NASA/ESA Hubble Space Telescope captured this revealing image of the galaxy cluster RDCS1252.9-2927.

The image shows the entire cluster (1/15 of a degree, corresponding to about 7 million light-years, across). The cluster probably contains many thousands of galaxies. Most of the other galaxies in the image, including most of the blue galaxies, are foreground or background galaxies. The image, which is made with an additional infrared exposure taken with the European Southern Observatory’s Very Large Telescope, shows mature galaxies in a massive cluster that existed when the cosmos was 5 billion years old. The cluster, called RDCS1252.9-2927, is as massive as 200 trillion suns and is the most massive known cluster for its epoch. Dominating the core are a pair of large, reddish elliptical galaxies [near centre of image]. Their red colour indicates an older population of stars. Most of the stars are at least 1 billion years old. The two galaxies appear to be interacting and may eventually merge to form a larger galaxy that is comparable to the brightest galaxies seen in present-day clusters.

The red galaxies surrounding the central pair are also cluster members.

The colour-composite image was assembled from two observations (through i and z filters) taken between May and June 2002 by the ACS Wide Field Camera, and one image with the ISAAC instrument on the VLT taken in 2002 (combined from a J filter exposure and a K filter exposure).

Credit: NASA, ESA, J. Blakeslee (Johns Hopkins University), M. Postman ( Space Telescope Science Institute) and P. Rosati, Chris Lidman & Ricardo Demarco (European Southern Observatory)