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2.1. COMBO-17: Galaxy-Galaxy Lensing

The acronym COMBO-17 stands for "Classifying Objects by Medium-Band Observations in 17 filters" [21], [22], [23]. The COMBO-17 survey consists of high-quality imaging data with the ability to obtain both rest frame colors and accurate photometric redshifts (deltaz / (1 + z) < 0.01 for R < 21, deltaz / (1 + z) ~ 0.02 for R ~ 22, and deltazphot < 0.1 for R < 24). The survey consists of 5 fields, including an extended region in the location of the Chandra Deep Field South (CDFS). The observations were carried out using the Wide Field Imager at the 2.2-m MPG/ESO telescope. The field of view of the camera is 34' × 33' and the 17-band filter set covers a wavelength range of 350 nm ltapprox lambdaobs ltapprox 930 nm. The latter allows for a rough determination of the spectral energy distributions of the objects, which in turn leads to both reliable classification of the objects into galaxies, quasars, and stars, as well as the ability to determine accurate photometric redshifts. A catalog containing astrometry, photometry in all 17 bands, object classification, and photometric redshifts for the 63,501 objects in the extended CDFS is publicly-available [23] (see The COMBO-17 results that will be discussed here consist of efforts to use galaxy-galaxy lensing to study dark matter halos. The data set is particularly well-suited to this task because of the reliability with which background galaxies (i.e., lensed sources) can be separated from foreground galaxies (i.e., the lenses). Note, too, that although the full COMBO-17 survey covers 5 fields, the results shown here come from only 3 of the fields (a field centered on the cluster A901, the CDFS field, and a random field [24]).

2.2. RCS: Galaxy-Galaxy Lensing

The RCS ([25], [26]) is a somewhat shallow (5sigma point source detection limits of RC ~ 24.8 and z' ~ 23.6), wide field (~ 90 sq. deg.) imaging survey that was designed primarily to search for galaxy clusters out to redshifts of z ~ 1.4. The images for the complete survey were obtained with the CFHT and CTIO 4-m telescopes using mosaic cameras, and consist of 22 widely-separated patches of ~ 2.1° × 2.3°. The RCS results that will be discussed here consist of galaxy-galaxy lensing studies and were obtained from ~ 42 sq. deg. of northern RCS data. Without spectroscopic or photometric redshift information, the RCS galaxy-galaxy results had to be obtained from a rough separation of lenses and sources that was based upon apparent magnitude cuts (i.e., galaxies with "faint" apparent magnitudes are on average background objects while galaxies with "bright" apparent magnitudes are on average foreground objects). Although the foreground-background distinction between a given pair of galaxies in the RCS data is by no means as secure as in the COMBO-17 data, the RCS is nevertheless a superb data set for galaxy-galaxy studies simply because of the area covered (~ 45 times larger than COMBO-17 for the weak lensing work). Given that weak lensing is primarily a statistical game, this is a good example of how well the galaxy-galaxy lensing signal can be detected and also used to constrain the nature of dark matter halos given only minimal distance information and a tremendous number of candidate lenses and sources.

2.3. SDSS: Galaxy-Galaxy Lensing & Satellite Dynamics

The SDSS is a combined photometric and spectroscopic survey that will ultimately map roughly one quarter of the sky above l ~ 30° and provide redshifts of ~ 106 galaxies and ~ 105 quasars with r' ltapprox 17.8. The SDSS is a fully-digital survey and makes use of 5 broad optical bands (u', g', r', i', z') for photometry. The data for the SDSS are being acquired at the Apache Point Observatory in Sunspot, New Mexico using a 2.5-m telescope, as well as three, smaller subsidiary telescopes for the purposes of photometric calibration, monitoring of the seeing, and scanning for clouds. The rms galaxy redshift errors are ~ 20 km sec-1to ~ 30 km sec-1(e.g., [27], [28]). A technical summary of the SDSS can be found in York et al. [29], information about the main galaxy sample is given by Strauss et al. [30], and information about the photometric system and photometric calibration is given by Fukugita et al. [31], Hogg et al. [32], and Smith et al. [33]. All of the SDSS data, including astrometry, photometry, redshifts, and spectra, are available via the SDSS website ( using structured queries that can search and combine the individual data bases. The third SDSS data release occurred on September 27, 2004 and includes spectra of 374,767 galaxies, spectra of 51,027 quasars, and photometry of 141 million unique objects. The SDSS results that will be discussed here consist of both galaxy-galaxy lensing studies and studies of the satellites of large, isolated galaxies.

2.4. 2dFGRS: Satellite Dynamics

The 2dFGRS is a spectroscopic survey in which the target objects were selected in the bJ band from the Automated Plate Measuring (APM) galaxy survey ([34], [35]) and extensions to the original survey. A detailed discussion of the survey and the data base is given by Colless et al. [36]. The observations, which are now complete, were carried out at the Anglo-Australian Telescope using the Two Degree Field (2dF) multifiber spectrograph. The final data release occurred on June 30, 2003 [37] and includes reliable redshifts of 221,414 galaxies with extinction corrected magnitudes of bJ geq 19.45, covering an area over ~ 1500 square degrees. Galaxies with reliable redshifts have an rms uncertainty of 85 km sec-1 [36]. All data, including spectroscopic catalogs (245,591 objects), photometric catalogs (382,323 objects), and FITS files containing the spectra, are publicly-available from the 2dFGRS website ( The 2dFGRS data base is fully-searchable via structured queries, and on-line documentation is available on the 2dFGRS website. The photometric transformation from the SDSS band passes to bJ is

Equation 1 (1)

(e.g., [38]). The 2dFGRS results that will be discussed here consist of investigations into the nature of the dark matter halos of large, isolated galaxies that are orbited by one or more satellite galaxies.

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