The Sloan Digital Sky Survey Quasar Catalog: fourteenth data release
DR14 Quasar Catalog
Following the tradition established by SDSS-I/II/III, the SDSS-IV/eBOSS collaboration produces quasar catalogs for public release. The SDSS-DR14 quasar catalog (DR14Q) was the first to be released that contains new identifications and redshifts that are mostly from eBOSS: it contains 526,356 quasars among which 144,046 are new discoveries. The DR14Q content is fully detailed in Pâris et al. (2018). Below, we summarize the main characteristics of DR14Q.
In order to provide the first direct measurement of the baryonic acoustic oscillations (BAO) scale at z ~ 1.5, SDSS-IV/eBOSS aims to uniformly select and successfully observe over 500,000 quasars in the redshift range 0.9 < z < 2.2, as well as recovering > 50,000 new quasars at z > 2.1 for Lyman-α forest measurements (see Myers et al. 2015 for details). Since clustering measurements require a catalog of maximal purity, quasar candidates suspected of having an incorrect pipeline redshift have been visually inspected. Note that DR14Q is not expected to be uniform, it is expected to be a more accurate census of quasars than using SDSS pipeline classifications and redshifts alone. The procedures necessary to obtain a uniform catalog for clustering studies are outlined in Section 3 of Ata et al. (2017).
Inspected quasar candidates
As the volume of SDSS-IV quasars is large compared to previous iterations of the SDSS, a new algorithm was applied for DR14Q in order to determine which quasars need to be visually inspected.
We start with the first five identifications produced by the SDSS spectroscopic pipeline (i.e. identifications corresponding to the five lowest values of reduced-χ2), the pipeline redshifts (zpipeline), and the ZWARNING quality flag. We then apply the following algorithm:
1. If the first SDSS pipeline identification is STAR, then the resulting classification is STAR;
2. If the first SDSS pipeline identification is GALAXY with zpipeline < 1, then the resulting classification is GALAXY;
3. If the first SDSS pipeline identification is GALAXY with zpipeline ≥ 1 and at least one other SDSS pipeline identifications are GALAXY, then the resulting classification is GALAXY;
4. If the best model fit for the observation is QSO and 2 or more model fits are STAR, the observation is classified as a STAR;
5. If the best model fit for the observation is QSO, fewer than two model fits are STAR, and ZWARNING = 0, the object is classified as a QSO;
6. If an observation meets none of these criteria it is selected for visual inspection (VI).
Each of these 5 criteria yield a classification (and redshift), which is retained as correct. If none of the 5 criteria is passed by an object, then the object is visually inspected. This approach results in ~7% of objects being visually inspected.
In SDSS-III/BOSS, the visual inspection was performed for all quasar candidates in order to (i) secure the identification of the objects, (ii) reliably estimate the emission redshift of quasars, and (iii) identify peculiar features such as Damped Lyman-α systems (DLA) and Broad Absorption Lines (BAL). We therefore manually confirmed or modified the identification of the object and, when needed, corrected the redshift provided by the pipeline, i.e. when it was wrong (when e.g. an emission line is misidentified or a bad feature is considered an emission line) or inaccurate (when emission lines are correctly identified but not properly centered).
This strategy changed for SDSS-IV: only quasar candidates that pass none of the criteria described above are visually inspected. During this residual visual inspection, BALs are no longer visually flagged and redshifts are corrected only in case of catastrophic failures of the SDSS pipeline.
After visual inspection, each quasar candidate is classified among one of the following categories: QSO, QSO_BAL, QSO_Z? when we know this is a quasar but its redshift is not certain, QSO_? when the object is possibly a quasar, Star, Star_? when the object is possibly a star, Galaxy, ? when the identification is uncertain or Bad when the data are not good enough to ascertain identification. The SDSS-DR14 Quasar Catalog only contains secure identifications (i.e. QSO and QSO_BAL).
We are making the final version of the DR14 quasar catalog (DR14Q_v4_4.fits) available. It should contain all of the quasars that comprise the SDSS-DR14 Quasar Catalog. Of order 526,000 quasars have so far been identified across the SDSS-I/II/III and IV surveys.
DR14Q_v4_4.fits (750 MB, fits format): Final SDSS-DR14 quasar catalog. The catalog is final. Compared to the previous version, 3 objects wrongly identified were removed. Missing coordinates were added. This version corresponds to the DR14Q paper (Pâris et al. 2018).
Previous versions of DR14Q
DR14Q_v3_1.fits (596 MB, fits format): Update of DR14Q_v3_0 without 377 quasars with null coordinates. Empty columns were also removed.
DR14Q_v3_0.fits (1 GB, fits format): Original SDSS-DR14Q quasar catalog.