Galaxies Near SDSS Quasar Sight Lines


Photo of Benjamin Weiner
Benjamin Weiner
University of Arizona


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 bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers Number of Unique Primary Objects
GAL_NEAR_QSO 62 A galaxy that lies between 0.006′ and 1′ of the line-of-sight for a spectroscopically-confirmed quasar 2,040 1,945


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.

This sample of approximately 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?
  • What is the size of velocity offsets?

Target Selection

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′.


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