No Quasar Left Behind


Photo of Nat Butler
Nat Butler
Arizona State University


An attempt to find all likely quasars in the 220-square-degree footprint of SDSS Stripe 82, selecting based only on variable source light curve data

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_VAR 3 Likely quasar selected by the No Quasar Left Behind program 1,510 1,394


We observed unresolved sources that had not been previously observed in SDSS spectroscopy that exhibit photometric variability statistically similar to that of spectroscopically confirmed quasars.

The adopted variability-based selection criteria correctly identify 96 percent of previously known quasars. Overall, than 80 percent of these ancillary targets are expected to be quasars in the redshift range 0 < z < 5.

Target Selection

Out of 11,000 variable sources with 16.2 < iPSF < 20.5, we selected 1,500 targets identified using the technique outlined in Butler & Bloom (2011). The sample complements the sample from the ancillary product Variability Selected Quasars, but targets brighter objects without color cuts, thus leading to a higher density of lower-redshift quasars.

The brightest of these targets (iPSF < 19) were observed to test the completeness of the color-selected SDSS sample (Ross et al. 2011). The fainter subset represents a nearly complete sample that was selected from uniform photometry. In addition, this ancillary target program provides an invaluable training sample for optimizing quasar selection algorithms based on photometric variability, which will be vital for planning future synoptic surveys.


Butler, N. R. & Bloom, J. S. 2011, AJ, 141, 93
Ross, A. J., et al. 2011, MNRAS, 417, 1350