Quasar Pairs


Photo of Adam Myers
Adam Myers
University of Wyoming


Spectra of closely-separated quasar pairs, taken to provide a large statistical sample of quasar pairs for studies of small-scale quasar clustering

Finding Targets

An object whose ANCILLARY_TARGET2 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
QSO_XD_KDE_PAIR 15 Candidate closely-separated quasar pair 645 628


Candidate quasar pairs separated by angles corresponding to less than a few 100 kPc were identified for spectroscopic confirmation. When combined with spectroscopy from other programs (e.g., Hennawi et al. 2006; Myers et al. 2008; Hennawi et al. 2010), this sample will provide a large statistical sample of quasar pairs necessary for small-scale clustering measurements.

Target Selection

The target list consists of pairs of quasar targets selected using either the KDE method (Richards et al. 2009) or the XDQSOz method (Bovy et al. 2011). There is a low-z selection and a mid-z selection, both identified by the QSO_XD_KDE_PAIR target flag.

The low-redshift selection includes targets with dereddened gPSF < 20.85 mag and a matching target from the same selection within an angular separation, θ, of 1″ < θ < 30″. Objects are selected based on being in the XDQSOz low-z selection range (0 < z < 2.2) probability being a quasar, PQSO > 0.8; or in the KDE catalog with flags indicating that the object is at low redshift and/or has an ultraviolet excess (lowzts = 1 or uvxts = 1, as described in Table 2 of Richards et al. 2009).

The mid-redshift selection includes XDQSOz targets with PQSO > 0:2 that have a pair (from the same mid-z
selection) within 1″ < θ < 20″. These targets are further culled to retain only pairs for which the product of the two XDQSOz probabilities for the pair, integrated over 2.0 < z < 5.5, is PQSO1 X PQSO2 > 0.16.

For both the low- and mid-redshift selection, the following algorithm is implemented to clean the sample:

  1. Target all pairs where one or both of the objects in the pair are in a BOSS tiling overlap region
  2. for pairs where both objects are outside overlap regions, target the object with no existing spectrum
  3. for pairs where both objects are outside of overlap regions and neither have existing spectra, target the fainter object
  4. Discard all pairs where both objects are outside of overlap regions and one of the pair is already a BOSS target
  5. Discard all pairs where either object is a spectroscopically-confirmed star or is obviously junk on visual inspection of imaging.
  6. Discard all targets (not pairs) that have an existing spectroscopic confirmation


Bovy, J. et al. 2011, ApJ, 729, 141

Hennawi, J. F., et al. 2006, AJ, 131, 1

Hennawi, J. et al. 2010, ApJ, 719, 1672

Myers, A. et al. 2008, ApJ, 678, 635

Richards, G. et al. 2009, ApJS, 180, 67