Quasar catalog

We present a catalog of photometrically selected quasars candidates with their corresponding photometric redshifts and redshift uncertainties. The catalog and methodology is described in Nakoneczny et al. (submitted, placeholder for an arXiv link).

The catalog results from applying artificial neural networks to process the KiDS data limited to 9- band detections. The machine learning (ML) models are trained on KiDS objects cross-matched with the spectroscopic SDSS survey. We address the problem of extrapolation to KiDS objects fainter than the SDSS limit by properly generalising ML models, and creating inference subsets which describe the reliability of estimations: safe, extrapolation, unsafe. We provide the suggested cuts on magnitude and probability of photometric classification, which are derived from validating the catalog with several methods. For details, please refer to Nakoneczny et al. (submitted).

Catalog format

Column nameDescription
RAJ2000 Centroid sky position right ascension (J2000)
DECJ2000Centroid sky position declination (J2000)
MAG_GAAP_rr-band GAaP magnitude with optimal MIN_APER (extinction corrected)
CLASS_STARSExtractor star-galaxy classifier
MASK9-band mask information
{CLASS}_PHOTOProbability that the source is in one of the three classes: GALAXY, QSO, STAR
CLASS_PHOTOObject class with the highest probability
Z_PHOTO_QSOPhotometric redshift for quasars
Z_PHOTO_STDDEV_QSOUncertainty of photometric redshift for quasars
SUBSETML inference subset (see Section 2.2 in the paper). Values: safe, extrapolation, unsafe


Data access

1. Catalog of quasar candidates
File: KiDS_DR4_QSO_candidates.fits
File size: 110 MB
Number of objects: 1,095,711
Data limited to:

  • 9-band detections
  • r < 25
  • CLASS_STAR < 0.2 or CLASS_STAR > 0.
  • QSO_PHOTO > 0.9
Possible values for the inference subset: safe, extrapolation.
Suggested cut for the extrapolation subset: 22 < r < 23.5 and QSO_PHOTO > 0.98.

2. Catalog of all machine learning estimates
File: KiDS_DR4_all_ML_estimates.fits (placeholder for a link when available at CDS; please email us to gain access before publicly available online)
File size: 5.5GB
Number of objects: 45,469,955
Data limited to 9-band detections.


Please cite Nakoneczny et al. (submitted) and include the KiDS DR4 acknowledgments.