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
(
z / (1 +
z) < 0.01 for R < 21,
z / (1 +
z) ~ 0.02 for R ~ 22, and
zphot
< 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 
obs
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 http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/421/913).
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
(5
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'
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
(http://www.sdss.org)
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 
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
(http://www.mso.anu.edu.au/2dFGRS).
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
 |
(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.