Spectroscopic discovery is by far the most prolific means of identification of Galaxy-Galaxy Lensing candidates, with follow-up Hubble Space Telescope (HST) imaging providing high-resolution, high signal-to-noise data for lens modeling, leading to results such as those of the Sloan Lens ACS (SLACS) survey, and ongoing discovery using the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey III (SDSS3).

The basic premise behind the spectroscopic selection of Bolton et al. 2004AstronJ127(2004)1860 was to find background emission lines within the same 3 arcsecond-diameter solid angle covered by the SDSS spectroscopic fiber remaining after subtracting best-fit galaxy templates to the target spectrum (lens candidate). Since the ratio of emission line wavelengths is independent of source redshift, the ratio of background emission lines must be identified at particular, known values. The procedure following Bolton et al. 2006ApJ638(2006)703, Bolton et al. 2008ApJ682(2008)964 looked for either resolved or blended background O_II doublets at 3727.092 Å and 3729.875 Å, respectively, times a factor of (1+z), where z is the computed redshift of the background emission line (source) galaxy. SLACS candidates were also required to exhibit two longer wavelength emission lines: Hβ at a wavelength of 4862.683 Å (1+z), and one (or both) of the O_III emission lines, at wavelengths of 4960.295 Å , or 5008.239 Å times (1+z), respectively.

Each SDSS image of candidate pairs was visually inspected to verify the absence of bright neighboring galaxies that could produce anomalous emission lines, and to determine the morphology of the lens galaxy -- SDSS images have been visually inspected to identify spiral galaxies sufficiently inclined for galaxy rotation curve measurements.

The probability of finding a galaxy-galaxy lens using the spectroscopic discovery method was considered in Dobler et al. 2008ApJ685(2008)57 for the SLACS survey, including selection effects such as the finite size of the spectroscopic fiber which selects against large separation lenses, and the effectiveness of spectral noise modeling which selects against sources that have redshifted emission lines coincident with strong emission lines in the sky. The finite wavelength range of the spectrographs places an upper limit on detecting high source redshifts.

Investigators may download reduced HST exposures

Strong gravitational lensing produces unmistakably distorted, amplified, and multiplied images of distant galaxies, and has led to the spectroscopic discovery of strong galaxy lenses by searching for evidence of emission from a background source (Bolton et al. 2004AstronJ127(2004)1860).

The Sloan Lens ACS (SLACS) survey uses the background emission line spectroscopic discovery method in combination with high-resolution, follow-up Hubble Space Telescope (HST) imaging, and is already an unprecedented sample of nearly 100 early-type galaxies and nearly 20 late-type galaxies, with completed multi-color HST imaging (Bolton et al. 2006ApJ638(2006)703, Bolton et al. 2008ApJ682(2008)964). Massive, early-type galaxies have been found to be homogeneous, with nearly isothermal density. However, variation in the dark matter fraction suggests a bulge--halo conspiracy, which seems to require a collisional scenario where gas and dark matter couple during galaxy formation (Kochanek 2005astro-ph/0412089, Treu et al. 2006ApJ640(2006)662, Koopmans et al. 2006ApJ649(2006)599, Koopmans et al. 2009aApJL703(2009)L51}. The current Baryon Acoustic Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey III (SDSS3), and upcoming next-generation digital sky surveys, should maintain the revolutionary progress of the SLACS survey and are expected to find ∼ 10⁴ new strong lenses in the current decade (Koopmans et al. 2009bApJL703(2009)L51).

For each SLACS lens, the redshift and velocity dispersion of the (foreground galaxy) lens and redshift of the (background galaxy) source are determined from the Sloan Digital Sky Survey (SDSS). Follow-up multi-color imaging, with the Hubble Space Telescope (HST), is used to model the lens and compute the aperture masses (within the Einstein radius) of the lens galaxy.

Investigators may access the HST Target Database

The SLACS survey has produced the largest selected sample of strong lenses, with 85 definite (and 13 probable) early-type galaxies. By combining strong-lensing aperture masses with lens-galaxy luminosities, sizes, and velocity dispersions, the SLACS collaboration has published the following results Koopmans et al. 2006ApJ649(2006)599, Gavazzi et al. 2007ApJ667(2007)176, Bolton et al. 2008ApJ682(2008)964, Koopmans et al. 2009aApJL703(2009)L51:

	The average total mass-density profile of early-type galaxies is nearly isothermal.
	Models for early-type galaxies in which light traces mass are falsified at > 99.9% confidence even within the optical half-light radius.
	The average weak lensing signal out to a few hundred kiloparsecs around the SLACS lenses is consistent with an isothermal extrapolation of the strong-lensing constraints from the inner few kiloparsecs
	The average density structure of elliptical galaxies is universal and does not vary systematically with mass, with the relationship between lensing mass and dynamical mass having a logarithmic slope of 1.03 ± 0.04.
	The structure of early-type galaxies exhibits a small but significant intrinsic scatter that is not significantly correlated with any other observable quantity.