Surveying the Distant Universe through Massive Clusters Lenses

Its true; light doesn't always travel in "straight" lines!

The gravitational wells associated with massive clusters of galaxies, like that shown above, act as "gravitational lenses" which focus light from background sources creating highly-magnified (and in rare cases multiple) images of very distant background sources. These images are sometimes referred to as gravitational arcs, since any distant galaxy seen through the lens is magnified (in a tangential direction) shearing the image of the galaxy into one or more `arclets'. The distortion of these arclets can then be used to trace the distribution of unseen material - dark matter perhaps - within the lens region.

To understand more about the distribution of mass within these massive galaxy clusters, Ian Smail (Durham University) and collaborators have undertaken a lensing survey with the Hubble Space Telescope. The gravitationally-lensed features identified in their HST exposures allow them to construct detailed mass maps for the central 1 mega-parsec (30 thousand million million million kilometer!) regions of these clusters. By incorporating wide-field imaging from other telescopes (like UKIRT) they trace the dark matter profiles of the clusters out to the point where they merge into the surrounding field.

The HST observations provide information not only about the mass distribution within the lens, but also about the redshifts of the background galaxies whose images are significantly magnified (and thus distorted) by the lens. The images of some of these background galaxies can be magnified by more than a factor of 10, thus offering an enormous increase in sensitivity for the detailed study of these faint, high-redshift galaxies. Indeed, by constructing an accurate mass model for the cluster, Dr Smail and his co-workers can "invert" it to predict redshifts for the faint galaxies seen in the background.

Dr Smail is now using UFTI on UKIRT to obtain high quality K-band imaging of gravitationally-lensed galaxies seen through the rich clusters in his HST survey. The observations above are of the X-ray luminous cluster Abell 383 (at z=0.19). The image comprises UFTI data combined with ground-based and HST optical images. The median seeing for the K-band observation was 0.42 arcsec, compared to 0.17 arcsec in the HST image and 0.88 arcsec in the Canada-France-Hawaii Telescope B-band exposure.

The most obvious lensed feature in the Figure is the giant "blueish" arc near the bottom of the image. This comprises four images of two background galaxies. Several other unusual lensed galaxies are visible, particularly in the HST component of the image. However, the UFTI imaging also turned up an extremely unusual galaxy seen through the core of the cluster, now referred to as an Extremely Red Object (or ERO). This galaxy - the red point bottom-left in the image - has a K-band magnitude of 19.7 and R-K of 6.0; this means that it is only just visible in the HST R-band exposure, but much clearer in the UKIRT data. EROs comprise a rare class of galaxies which are believed to consist of a mixture of evolved early-type galaxies at z=1-2 and dusty starburst galaxies at similar (and higher) redshifts. However, by definition EROs are faint at optical wavelengths and hence only a couple have reliable redshifts measurements. This lack of information about their redshifts and hence luminosities and true "restframe" colours has so far hampered efforts to disentangle the properties of this unusual phenomenon.

Our thanks to Ian Smail and collaborators for allowing us to reproduce their data.

Image Credits: The UFTI, HST and CFHT imaging of this cluster was taken for a collaborative study by our group. These observations were obtained with UKIRT, which is operated by the Joint Astronomy Centre on behalf of the Particle Physics and Astronomy Research Council of the United Kingdom; CFHT, operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii and the NASA/ESA Hubble Space Telescope, which is operated by the Space Telescope Science Institute for the Association of Universities for Research in Astronomy Inc., under NASA contract NAS 5-26555.