Clusters as gravitational lenses

Gravitational lensing by clusters of galaxies is recognized as a powerful cosmological tool. By measuring the various properties of the gravitationally magnified and distorted mirages of background galaxies, important information can be learned about both the lensing cluster and the faint, distant, lensed background galaxies. Lensing is also a very useful application as a tool for studying the clusters themselves. It can be used to measure the cluster mass, independent of assumption about virialization of the galaxies or hydrostatic equilibrium of the hot X-ray emitting intracluster gas. Gravitational lensing provides a means to explore the actual shape of a cluster's total mass distrubution, both visible and dark matter, at small radii (r<500kpc) using strong gravitational lensing (giant arcs), and at large radii using weak gravitational lensing. It is well known that projection and contamination problems can be avoided if clusters are selected based on their primary baryonic constituent - the hot, intracluster gas that is a copious emitter in X-rays - rather than on the optical galaxies which are merely trace components. Both MS0440+0204 and MS2137-2353, shown below, were detected as X-ray sources in the EMSS. See Luppino, Gioia et al. (1999) for a search for gravitational lensing in 38 EMSS clusters of galaxies, and Clowe et al. (1998) for weak gravitational lensing of distant background galaxies by two rich, X-ray-luminous clusters of galaxies, MS 1137+66 (EMSS cluster) at z = 0.783 and RXJ 1716+67 (NEP cluster) at z = 0.813.

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EMSS clusters with giant arcs

The composite picture shows seven out of eight EMSS clusters with giant arcs. The CCD imaging survey for gravitational lensing was carried out with the University of Hawaii (UH) 2.2 m telescope and the Canada France Hawaii telescope (CFHT) on Mauna Kea. The sample consists of the most X-ray luminous (Lx > 2x10^44 erg/s) EMSS clusters that are observable from Mauna Kea (declination > -40deg), and spans a redshift range of 0.15 < z < 0.823 with 5 clusters at z > 0.5. CCD images of the clusters were obtained in excellent seeing. Evidence for strong gravitational lensing in the form of giant arcs (length l > 8'', axis ratio l/w > 10) is present in 8 of the 38 clusters. Images are published in Luppino, Gioia, Hammer, Le Fevre & Annis, 1999.