IntroductionThe sky contains hundreds of thousands of foreground massive objects, either massive galaxies, Dark Halos, galaxy clusters (Miralda-Escude 1992, 1993a, 1993b and Marshall et al. 2005), which, when intercepting the line-of-sight of a galaxy in the background, produce multiple virtual images of that galaxy. They are, then, called Strong Lenses. Historically, strong lenses are classified into three varieties; giant arcs (actually multiple images of an extended object merged together; Soucail et al. 1987, Fort et al. 1988), multiple arclets (e.g. radial pairs; Fort et al. 1992) and gravitational rings (a rare occurence when both the foreground lens and the background source share the same line-of-sight; Warren et al. 1996 observed the first optical ring; also Cabanac et al. 2005).
Because the optical depth of geometrical alignment events span
10-5 to 10-3 (Oguri et al. 2004, and reference therein), future spaceborne missions like DUNE (cf. pdf presentation by A. Refregier on
SNAP will discover
about a dozen of lenses per square degree. This field of studies will
thrive in the era of large space telescopes
In this perspective a great deal of theoretical works have surged recently
(see next section below).
Science rationaleStrong Gravitational lenses give a unique tool to probe directly the projected mass profile of any type of objects, such as galaxies (Kochanek et al 2001, Keeton 1998, 2001, McLeod 2001), compact groups or galaxy clusters (Pello et al. 1991, Mellier et al 1993, Fort & Mellier 1994, Kneib et al 1993, 1994, , 2000, 2003). Strong lensing has already led to remarkable premieres; first constraint on the upper value of the cosmological constant (Λ<0.9) for a flat universe (Kochaneck et al. 1992); first discovery of very high mass concentration in the center of galaxy clusters (Mellier et al. 93); The direct measure of the galaxy mass within the Einstein radius; The detection of the farthest objects in the universe (Pello et al. 2005, 2003, Kneib et al., 2004), to mention only the most spectacular. As impressive as past works might seem, it only represent a small token of the potential of Strong Gravitational Lensing, mostly because past samples (principally made of arcs in clusters and multiply imaged quasars) were very small. In addition, most discoveries were serendipitous in a wide range of wavelengths, resolutions, seeings, fields, hence sorting systematics and completeness of that sample was a formidable task. For instance, up to 2005, the largest sample homogeneous sample the Cosmic Lens All-Sky Survey (CLASS) comprising multiple images quasars, had only 22 strong lenses. A major survey of compact lenses detected spectroscopically on the SDSS is currently ongoing; The Sloan Lens ACS Survey (SLACS) already contains 28 targets and shall reach nearly 100. Due to the selection on the SDSS spectroscopic survey, the SLACS sample is restricted to deflectors at low redshift (z<0.3). In comparison, a preliminar scan of the first fields of the CFHTLS WIDE (172 deg2) shows that the final sample shall approach 200 ±30 giant arcs and at least twice that number of small rings and multiple arclets around massive galaxies and groups of galaxies. Hence, the CFHTLS potentially represents the largest database of Strong Lensing events in the coming 5 years.
The science realm the CFHTLS-SL2S can probe is vast, and our first priorities
will be to characterize the deflectors (lenses) and the lensed sources.
Constraints on cosmological parameters can certainly be approached by
the CFHTLS-SL2S, but strong constraints down to levels of a few percents,
which is required for post SN-WMAP dark energy cosmography, demand
larger samples only available through space missions
However, our detection techniques are very generic and shall allows us to
discover new types of Strong Lenses, such as dark lenses. In the
short term, we are interested in the following axes of research: