Variability-selected Quasars at 1 < z < 4 to g = 22.5


Photo of Nathalie Palanque-Delabrouille
Nathalie Palanque-Delabrouille
Institute of Research into the Fundamental Laws of the Universe
CEA Saclay


Spectra of quasars in SDSS stripe 82, selected by color and variability

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 Spectra Unique Primary Objects
QSO_VAR_LF 27 Candidate quasar in stripe 82, selected by color and variability 2,400 2,328


The target flag QSO_VAR_LF labels a target class designed for studies of the quasar luminosity function to g < 22.5. The sample is located in Stripe 82 at 36° < α < 42° where multi-epoch SDSS photometry is available, thus enabling a variability selection with the neural network presented in Palanque-Delabrouille et al. (2011).

Target Selection

Targets with pointlike morphology that passed a loose variability criterion were selected (neural network threshold of 0.5, where 1/0 indicates a quasar-like/stellar-like light curve). Extended sources which satis ed the color selection c3 < 0.6 – 0.33c1, where c1 and c3 are linear combinations of SDSS ugriz as defined in Fan (1999), were targeted if they passed a tighter variability criterion (threshold of 0.9).

Note that targets previously spectroscopically identified as quasars were not included in the sample, and therefore do not have the QSO_VAR_LF bit set, even if they pass the selection criteria for this program.


Fan, X. 1999, AJ, 117, 2528

Palanque-Delabrouille, N., et al. 2011, A&A, 530, 122