In early 1999 astronomers from the U.K. used the then-new UFTI camera on UKIRT to obtain high quality panoramic near-infrared images of galaxies within the rich cluster Abell 851 (at z=0.41). This very distant cluster is the subject of an intensive observational campaign with the Hubble Space Telescope (HST) and other ground-based telescopes. The colour image shown above was created from high-resolution I- and K-band images. The I-band data were obtained with the Keck telescope; the K-band data are from UKIRT (note that the bright stellar halos and bleeding only effect the Keck data). At the redshift of this cluster, the image spans 10,000 million light years!

Old, bright elliptical galaxies are clearly visible in the cluster centre, while the bluer star-forming galaxies are spread more widely across the field and probably represent an infalling population of star-forming field galaxies. The subsequent evolution of this population will have a profound effect on the mix of galaxies within the cluster core in the distant future.

Below a true colour image of the central region from the A851 UFTI mosaic is shown. This was created from R-, I- and K-band images, taken with HST, the Keck telescope and UFTI at UKIRT respectively. The image shows a compact clump of galaxies on the edge of the cluster which are possibly falling towards the cluster core. The effects of dust are evident in the red colours of several of the spiral galaxies in the centre of the field, as well as being seen in their HST morphologies. A faint, extended source seen only with UFTI in the K-band is visible bottom-right of the field centre; this is undetected in the deep optical (R and I) images giving it a very red colouration.

These data are being analysed to study the morphologies and properties of different types of galaxies; actively star-forming galaxies, post-starburst galaxies and passive galaxies. How do their properties change as a function of radius within the cluster? What are the mechanisms responsible for the rapid evolution of the galaxy populations in these environments? How have the characteristics of galaxies within rich clusters altered over the past 5 billion years? These are just some of the questions being addressed by astronomers. Clearly, UKIRT can play a very important role in this exciting field of astronomy, allowing researchers to obtain profound insights into the rules governing the formation and evolution of galaxies.

Data courtesy of Ian Smail (Durham University) and Rob Ivison (University College London). The above notes were adapted from an article that appeared in the UKIRT Newsletter in September 1999, written by Dr Smail and Dr Ivison.

Image credits: The UFTI image was taken for a collaborative study by Dr Ian Smail (Durham), Dr Rob Ivison (UCL), Dr Alan Dressler (OCIW) and Dr Bianca Poggianti (Padova). The HST mosaic comes from the MORPHS group and the Keck image was taken in collaboration with Professor Len Cowie and Dr Amy Barger of the University of Hawaii.