The SDSS Discovery of a Strongly Lensed Post-Starburst Galaxy at z=0.766 Page: 4 of 22
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from the imaging data. The properties of all objects detected above 5 a in at least one band
over the - 104 deg2 imaged to date are measured by a dedicated pipeline (Lupton et al.
2002) and are calibrated photometrically (Tucker et al. 2006) and astrometrically (Pier et al.
2003). We use the fifth public data release (Adelman-McCarthy et al. 2007) in selecting
lens candidates. Since most possible lensing galaxies are early-type galaxies because of their
large velocity dispersions (Turner et al. 1984; Fukugita & Turner 1991), we assume that
lensing galaxies are red galaxies in the SDSS images. Because only the most luminous of
those possible lensing galaxies have redshifts above 0.2 (Strauss et al. 2002; Eisenstein et al.
2001), we do not restrict our analysis to the sample of objects with redshifts.
Our candidate selection method is still being developed, so we only outline the criteria
that were used in finding the reported lens system here. First, we identify all red galaxies in
the photometric catalog that have a model magnitude in g-band < 23.0 without correction
for Galactic extinction. Red galaxies are defined using color cuts suggested by Goto et al.
(2002):
-0.2 < (r - i) - 0.25(g - r) - 0.18 <0.2, (1)
-0.2 < (g - r) < 2.0, (2)
where we use model magnitudes throughout. This color cut selects intrinsically red galaxies
up to redshift 0.4. We next select those red galaxies that have at least two photometric
companions between 2'"4 and 3'"6 away, a typical radius of strongly lensed systems, and a
large enough separation that the image deblender separates the lens images from the red
galaxy.
In order to discriminate a lensing system from a galaxy group or cluster, we require
that the u - g colors of the possible lens images differ by more than two magnitudes from
that of the lensing galaxy. This color difference is much larger than the observed color
range of cluster ellipticals (e.g. Goto et al. 2002). This color difference cut was adopted
to test our preliminary selection method and to reduce possible contamination of neighbor
galaxies around red galaxies. While cluster ellipticals are red and tend to be undetected in
u, background lensed galaxies are often quite blue and have very different u - g colors. We
discuss refinement of this color cut in 5.
In most strong quadruple lensing systems we expect to find at least two bright knots
on one side of the lensing galaxy '. We therefore require that the angle between the two
companion objects with respect to the possible lensing galaxy be less than 900. Even though
the surface brightness of the arc may not be high enough to appear in the SDSS images,'The case of double images is discussed in 5.
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Shin, Min-Su; Strauss, Michael A.; Oguri, Masamune; Inada, Naohisa; Falco, Emilio E.; Broadhurst, Tom et al. The SDSS Discovery of a Strongly Lensed Post-Starburst Galaxy at z=0.766, article, September 30, 2008; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc899378/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.