The central regions of the rich cluster of galaxies Abell 2218 (z=0.18). This is false-color representation of a three orbit HST/WFPC2 F702W (red) image, the field size is roughly 40×120 arcsec. There is an obvious population of tangentially aligned images (called `arcs') around both the central galaxy and its bright companion. These are gravitationally lensed images of background field galaxies, the distortion induced in their images (see the lower image) holds clues to both their distance behind the cluster and also the distribution of mass within the cluster lens. Using the observed shapes of these arcs an accurate model has been constructed for the dark matter in the cluster and this model has then been applied to determine the distances to a large sample of very faint field galaxies - far beyond the reach of conventional spectroscopy on even 10-m class telescopes. The accuracy of these predictions has been recently confirmed using spectroscopic observations of some of the brighter arclets. In particular a star-forming galaxy with a predicted redshift of z=2.8±0.3 was confirmed to lie at z=2.515.

A simulation showing how Abell 2218 would appear if mapped at 850um using the new SCUBA sub-mm array on the JCMT telescope, Mauna Kea. This image represents the equivalent of 12 hours integration on source and illustrates the strong constrast between the quiescent cluster ellipticals and the star-forming galaxies seen as giant arcs through the cluster lens. Such observations will provide strong constraints on the nature of galaxies in the distant Universe. For REAL SCUBA observations of lensing clusters see this page.

An HST/WFPC2 F702W (red) image of the core of the cluster AC114 (z=0.31). A number of multiply-imaged background galaxies are visible in this image, they appear as distinct images showing mirror symmetry. Two striking examples lie to the right of the central galaxy near the edge of the frame, including two images of a knotty `C'-shaped feature. A detailed mass model of the cluster lens has been constructed and indicates that the this galaxy probably lies at z=2.3. The brightest multiply-imaged feature in this field lies to the upper-right of the central galaxy - two compact images with mirror-symmetric tails. A zoom of these features is shown below. Other images of this cluster can be found here.

A zoom into the image shown above to demonstrate the high symmetry of one of the multiply-imaged sources. This object has been spectroscopically identified as a z=1.86 star-forming galaxy. The positions, orientations and relative brightnesses of these images have been used to accurately model the distribution and amount of dark matter in the central regions of the cluster. The observations of this feature are discussed in more depth here.

JPK's new identification of object D1-D5 in AC114. There are now three 5-image systems (A1-A5, B1-B5 and D1-D5) and two 3-image systems (S1-S3 and C1-C3). Although two of the 5-image configurations are just slightly perturbed 3-image systems, where masses associated with individual cluster galaxies have created extra images. Note: the labelling on the figure is old - Q1-Q3 are S1-S3 and the images A4 and B3 have been redesignated to show they are both double images.

A zoom in on the core of AC114 to show more detail in the various multiply-imaged sources. In particular the peculiar morphology of the source seen as C1-C2-C3 - this is believed to lie at z=2.1 and is clearly a very elongated galaxy with a ridge of strong star-formation. The higher surface brightness source B1-B5 shows much less structure and has a predicted redshift of z=1.2, while the clumpy object seen as A1-A5 is expected to lie at z=1.7. Therefore, along with the spectroscopically-identified galaxy S1-S3, this small region of the sky contains at least 4 high redshift galaxies each of which is being multiply-imaged. The high density of these distant galaxies attests to the strong amplification arising from and the high mass of the foreground lensing cluster.

A series of color image of AC114 (z=0.31) taken with the AAT by Warrick Couch and using data from the Danish 1.5m at La Silla. The frames are 2.0×3.5 arcmin and are made from deep B, V and I images. The central panel shows the true color BVI image, the panel to the left shows an image made with (B-V) and (V-I) exposures as `blue' and `red'. These color-subtracted images were scaled to remove the cluster ellipticals from the individual exposures. The panel to the right shows the reversed (V-B) and (I-V) exposures. In this panel anything which is bluer than the cluster ellipticals in (V-I) and redder in (B-V) will appear as `yellow'.

The 17 non-stellar objects with I<21 and (B-I)>4.2 selected from the above cluster. This color limit is applicable to a non-evolved elliptical SED at z>0.6. The three of these which lie on the HST field are confirmed as non-stellar, the four red stars that lie in this region are also correctly identified. Each subframe is 27 arcsec wide (and these are rotated by 90 deg ccw compared to the field above). Objects at [4,1], [1,2] and [3,2] lie on WFPC2 field and are non-stellar (see below), object at [5,3] lies off the HST field but is a true drop-out with (B-I)>5.5.

Images of the 7 red objects lying on the WFPC2 field. The bottom row has the four stars (the first one may be slightly non-stellar), the top row has the three galaxies listed above in the same order. The scale is in arcseconds.

A true-color image of the lensed features in the distant cluster Cl0024+16 (z=0.39). Five images of a blue background galaxy are visible around the cluster center (3 at the top of the frame, 1 at the bottom and 1 in the center between the 4 bright cluster galaxies), this object appears to be a ring system with a number of bright knots around the ring. The identification of an image of the background galaxy in the central regions of the cluster is crucial for understanding the distribution of dark matter in the cluster core.

The compact cluster Cl2244-02 at z=0.33 imaged with HST/WFPC2 in F555W/F814W. These images were combined to make the `true-color' image displayed below. The target of this observation was the bright semi-circular arc seen to the left of the cluster core, this has been spectroscopically identified as a z=2.235 galaxy, at the time the highest redshift `normal' galaxy known. The giant arc actually comprises two highly distorted images of the background galaxy which is thought to be a face-on spiral (see zoom). The two images have mirror symmetry and the relative magnifications and features shown by the two halves have led to a detailed understanding of the dark matter profile in the very central regions of this cluster. A number of other lensed features are visible around the frame.

A true-color image of the distant cluster A2390 (z=0.23), formed from F555W and F814W exposures. The central cluster galaxy lies slightly to the left of center in the field, a faint red arc can be seen around this galaxy as well as a number of bluer multiply-imaged sources. The wide elongated feature lying next to the second brightest cluster galaxy (center-right) is an lensed image of z=0.913 spiral galaxy, part of a group of galaxies lying at this redshift which can be identified by their yellow colors.

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Last Modified: April 25th, 1997. [Netscape 2.0]