Most gravitational lenses are identified through targeted discovery programs like GEMS (Galaxy Evolution from Morphologies and SEDs), which specifically aim to find and study these phenomena. Additionally, many lenses are discovered within the vast datasets collected from established surveys such as the Sloan Digital Sky Survey (SDSS). These surveys provide extensive imaging and spectroscopic data, allowing researchers to identify lensing events and study the characteristics of both the lensing objects and the background sources. Together, targeted programs and large-scale surveys significantly enhance our understanding of gravitational lensing and contribute to the discovery of new lenses.
Bells Gallery
The study presents a detailed morphological analysis of 17 lensed Lyα emitter (LAE) galaxies from the BELLS GALLERY sample, using the strong magnification effect of galaxy–galaxy lensing and the high resolution of the Hubble Space Telescope. This approach achieves a physical resolution of about 80 pc for LAEs in the redshift range 2 < z < 3, allowing precise characterization of their rest-frame ultraviolet continuum surface brightness profiles and substructure. Lens-model reconstructions were used to identify and model individual clumps, leading to the development of a generative statistical model of the LAE population. The study found a clumpiness fraction of approximately 88%, higher than previous non-lensing studies, and characteristic clump half-light radii of about 350 pc, similar to large H ii regions in the local universe. This statistical model of LAE surface-brightness profiles will aid future lensing analyses to study dark-matter substructure in foreground lensing galaxies.
SLACS
The Sloan Lens ACS (SLACS) Survey is an efficient Hubble Space Telescope (HST) Snapshot imaging survey aimed at discovering new galaxy-scale strong gravitational lenses. The lens candidates are selected from the Sloan Digital Sky Survey (SDSS) database based on the presence of multiple nebular emission lines at higher redshifts than the target galaxies. The survey is designed to detect bright early-type lens galaxies with faint lensed sources, enhancing the sample of gravitational lenses for detailed modeling.
In the first paper of the series on HST Cycle 13 imaging survey results, a catalog of 19 newly discovered gravitational lenses is presented, along with nine other observed candidate systems classified as possible lenses, nonlenses, or nondetections, resulting in a survey efficiency of at least 68%. Additionally, integral-field spectroscopic data from the Gemini 8 m and Magellan 6.5 m telescopes for nine SLACS targets support the lensing interpretation.
A new method for effectively subtracting foreground galaxy images to reveal faint background features is introduced. The SLACS lens galaxies exhibit colors and ellipticities typical of their SDSS parent sample (luminous red galaxies and quiescent MAIN sample galaxies) but are somewhat brighter and more centrally concentrated. Several reasons for this brightness bias are proposed.
The SLACS survey offers the first statistically significant and homogeneously selected sample of bright early-type lens galaxies, providing a valuable probe into the structure of early-type galaxies within the half-light radius. The high confirmation rate of lenses in the SLACS survey supports the idea of incorporating spectroscopic lens discovery as a specific goal in future spectroscopic galaxy surveys.
GEMS
GEMS: Galaxy Evolution from Morphologies and SEDs
GEMS (Galaxy Evolution from Morphologies and SEDs) is a comprehensive imaging survey conducted using the Advanced Camera for Surveys on the Hubble Space Telescope. Covering a large area of 800 arcmin² in two colors (F606W and F850LP), it is centered on the Chandra Deep Field-South and spans about 28′ × 28′, equivalent to approximately 120 Hubble Deep Field areas. The survey reaches a depth of mAB(F606W) = 28.3(5 σ) and mAB(F850LP) = 27.1(5 σ) for compact sources.
In its central quarter, GEMS includes ACS imaging from the GOODS project. Targeting galaxies within the redshift range 0.2 ≲ z ≲ 1.1, GEMS provides detailed morphologies and structural parameters for nearly 10,000 galaxies, leveraging redshift estimates, luminosities, and spectral energy distributions (SEDs) from COMBO-17. Additionally, GEMS captures images of host galaxies for several hundred faint active galactic nuclei.
This paper offers an overview of GEMS, covering its scientific objectives, experimental design, data reduction processes, and the planned scientific analysis.
SDSS
The Sloan Digital Sky Survey (SDSS) is a significant multi-spectral imaging and spectroscopic redshift survey conducted using a dedicated 2.5-meter wide-angle optical telescope at Apache Point Observatory in New Mexico, USA. The project commenced in 2000 and is named after the Alfred P. Sloan Foundation, which provided substantial funding.
The survey is carried out by a consortium led by the University of Washington and Princeton University, which formed to execute the redshift survey. The Astrophysical Research Consortium (ARC), established in 1984, includes additional participation from New Mexico State University and Washington State University to oversee operations at Apache Point. In 1991, the Sloan Foundation awarded the ARC funding to support the survey efforts and the construction of the necessary equipment.