[Paper Review] The CFHTLS Strong Lensing Legacy Survey: I. Survey overview and T0002 release sample

R. A. Cabanac, C. Alard+11 more|ArXiv.org|Oct 12, 2006

Advanced Optical Sensing TechnologiesPhysics and Astronomy44 references101 citations

TL;DR

This paper presents the CFHTLS Strong Lensing Legacy Survey (SL2S), an automated survey using deep, wide-field CFHTLS imaging to detect strong gravitational lenses across galaxy, group, and cluster mass scales. It reports a preliminary sample of ~40 lensing candidates—mainly arc systems with splitting angles of 2–15 arcsec—demonstrating for the first time the feasibility of discovering a large, statistically robust population of intermediate-mass lenses (10^13 h^{-1} M_{\odot}) from galaxy groups.

ABSTRACT

AIMS: We present data from the CFHTLS Strong Lensing Legacy Survey (SL2S). Due to the unsurpassed combined depth, area and image quality of the Canada-France-Hawaii Legacy Survey it is becoming possible to uncover a large, statistically well-defined sample of strong gravitational lenses which spans the dark halo mass spectrum predicted by the concordance model from galaxy to cluster haloes. METHODS: We describe the development of several automated procedures to find strong lenses of various mass regimes in CFHTLS images. RESULTS: The preliminary sample of about 40 strong lensing candidates discovered in the CFHTLS T0002 release, covering an effective field of view of 28 deg$^2$ is presented. These strong lensing systems were discovered using an automated search and consist mainly of gravitational arc systems with splitting angles between 2 and 15 arcsec. This sample shows for the first time that it is possible to uncover a large population of strong lenses from galaxy groups with typical halo masses of about $10^{13}h^{-1}M_\odot$. We discuss the future evolution of the SL2S project and its main scientific aims for the next 3 years, in particular our observational strategy to extract the hundreds of gravitational rings also present in these fields.

Motivation & Objective

• To develop and deploy automated detection pipelines for strong gravitational lenses across a wide dynamic range of lens masses, from isolated galaxies to massive clusters.
• To overcome limitations of previous surveys by identifying a statistically complete sample of lenses spanning redshifts up to z ≈ 1, including previously underexplored intermediate-mass systems.
• To enable detailed studies of dark matter halo structure, galaxy mass-to-light ratios, and the evolution of the fundamental plane through high-redshift lensing systems.
• To optimize detection of Einstein rings and multiplets, extending the reach of strong lensing beyond the local universe and improving constraints on baryonic physics and mass assembly.

Proposed method

• Utilizes deep, wide-field imaging from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) with MegaPrime/MegaCam, achieving high image quality and uniform depth over 170 deg².
• Employs three automated detection pipelines: an arc detector for giant arcs and arclets, a ring detector for compact Einstein rings, and a multiplet detector for systems with multiple images regardless of deflector morphology.
• Applies photometric and morphological selection to identify lensing features based on symmetry, multiple image configurations, and characteristic ring or arc morphologies.
• Leverages the T0002 data release (28 deg²) to validate detection algorithms and produce an initial sample of ~40 lensing candidates.
• Uses follow-up HST observations (Cycle 15, snapshot program 10876) to confirm the most promising candidates, particularly those with compact Einstein rings.
• Calibrates detection efficiency and selection effects through simulations and comparison with existing samples, including SLACS and COSMOS ring candidates.

Experimental results

Research questions

• RQ1 Can a large, statistically complete sample of strong gravitational lenses be identified across the full mass spectrum—from galaxies to clusters—using automated detection in deep wide-field surveys?
• RQ2 What is the abundance and distribution of intermediate-mass lenses (e.g., galaxy groups with M ≈ 10^13 h^{-1} M_{\odot}) in the CFHTLS data, and how do they compare to theoretical predictions?
• RQ3 How do the properties of Einstein rings and multiple-image systems evolve with redshift, and what can they reveal about the mass distribution and star formation history in lensing haloes?
• RQ4 To what extent can strong lensing constraints break degeneracies in dynamical modeling, particularly for the fundamental plane and mass-to-light ratios of early-type galaxies?
• RQ5 Can the detection of hundreds of rings in the full CFHTLS survey (170 deg²) enable robust statistical studies of halo structure and evolution beyond the local universe?

Key findings

• The SL2S survey successfully identified a preliminary sample of approximately 40 strong lensing candidates in the T0002 release, covering 28 deg² of the CFHTLS wide survey.
• The sample includes a significant number of intermediate-mass lensing systems, with halo masses around 10^13 h^{-1} M_{\odot}, indicating that galaxy groups are a major source of strong lensing features.
• The detection of arc systems with splitting angles between 2 and 15 arcsec confirms the feasibility of identifying lensing features across a broad range of lens masses.
• The ring detection pipeline, though not yet fully optimized, is projected to identify around 400 Einstein rings in the full 170 deg² CFHTLS wide survey, with ~300 expected to be robust after HST confirmation.
• The survey is expected to increase the known sample of strong lenses by a factor of 3 over a redshift range of 0.3 to 1, significantly extending the reach of lensing studies beyond the local universe.
• The project is projected to deliver a final sample of about 500 lenses by 2008, with well-controlled selection effects, enabling statistical studies of lensing geometry and halo mass profiles across the full mass scale.