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Galaxies Near SDSS Quasar Sight Lines


Spectra of galaxies in the SDSS Data Release 7 imaging area (7,650 deg2) which are close on the sky to known quasars, meaning that they lie along the quasar's line-of-sight

Finding Targets

An object whose ANCILLARY_TARGET1 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS-III bitmasks to learn how to use these values to identify objects in this ancillary target program.

(bit name)
Bit Target Description Target density
GAL_NEAR_QSO 62 A galaxy that lies between 0.006' and 1' of the line-of-sight for a spectroscopically-confirmed quasar 0.3


Obtaining accurate redshifts of galaxies that are projected on the sky near to the lines-of-sight to quasars allows for detailed studies of the properties of galaxies that are associated with intervening quasar absorption systems. Such systems could not be studied with the SDSS-I/-II spectrograph due to fiber collisions, but new the BOSS spectrograph makes it possible to spectroscopically observe objects at close separations.

Primary contact

Benjamin Weiner
University of Arizona
bjw -at-

Other contacts

Christy Tremonti, Jeremy Tinker, Jill Bechtold, Patrick Petitjean, Tim Heckman, Brice Ménard

Target selection details

Galaxy targets are selected by a ugr color cut to lie in a redshift range (z > 0.35), where the 2800 Å MgII line is detectable in SDSS spectra. The sample of spectroscopically-confirmed quasars used for target selection in this program was selected to have model magnitudes g < 19.2 and redshift 0.7 < z < 2.1 from the SDSS DR7 quasar catalog (catalog in Schneider et al. 2010a and source paper in Schneider et al. 2010b). Galaxies were chosen that lie between 0.006' and 1' of a quasar with existing SDSS spectroscopy. To photometrically select galaxies in the target redshift range (z > 0.35), galaxies were selected with model magnitudes 17.5 < i < 19.9, Ag < 0.3, and [(g-r) > 1.65 OR (u-g) < 1.14(g-r)-0.4].

The sample is weighted to have similar numbers of galaxies at separations b < 0.5' and 0.5' < b < 1'. This sample of approximately 3,000 galaxies will help answer questions such as: is absorption is correlated with galaxy type?; What is the covering fraction of absorption as a function of galaxy radius?; and what is the size of velocity offsets?


Schneider, D.P. et al. 2010a, catalog, bibliographic code: 2010yCat.7260....0S
Schneider, D.P. et al. 2010b, AJ, 139, 2360, doi:10.1088/0004-6256/139/6/2360