A photometric catalogue of quasars and other point sources in the Sloan Digital Sky Survey

Abstract

We present a catalogue of about six million unresolved photometric detections in the Sloan Digital Sky Survey (SDSS) Seventh Data Release, classifying them into stars, galaxies and quasars. We use a machine learning classifier trained on a subset of spectroscopically confirmed objects from 14th to 22nd magnitude in the SDSS i band. Our catalogue consists of 2 430 625 quasars, 3 544 036 stars and 63 586 unresolved galaxies from 14th to 24th magnitude in the SDSS i band. Our algorithm recovers 99.96 per cent of spectroscopically confirmed quasars and 99.51 per cent of stars to i ∼ 21.3 in the colour window that we study. The level of contamination due to data artefacts for objects beyond i = 21.3 is highly uncertain and all mention of completeness and contamination in the paper are valid only for objects brighter than this magnitude. However, a comparison of the predicted number of quasars with the theoretical number counts shows reasonable agreement.

XMM-Newtonstudy of the Draco dwarf spheroidal galaxy

“…The source is located in the region of quasars based on the colours of the SDSS7, but the u band magnitude of the SDSS9 survey is significantly different from that of SDSS7. With the colours of SDSS9 (u − g ≈ 1.9 and g − r ≈ 0.7), the source is located in the region of stars based on the colour-colour diagrams of Abraham et al (2012). On the other hand, the optical counterpart is located far from classified AGNs in colour−magnitude diagram (Fig.…”

“…AGN catalogues: we used the most updated catalogues of AGNs covering the field of the Draco dSph: the quasars and active galactic nuclei catalogue (13th edn., Veron-Cetty & Veron 2010b), the QSO candidates selection in the virtual observatory era (D'Abrusco et al 2009), the photometric classification catalogue of SDSS DR7 (Abraham et al 2012), the large quasar astrometric catalogue 2 (LQAC-2, Souchay et al 2012), the photometric selection of quasars from SDSS II (Richards et al 2009), the all-sky atlas of radio/X-ray associations (Flesch 2010), and the quasar catalogue of Kinemuchi et al (2008) in the field of the Draco dSph.…”

“…Source No. 24 was classified as a candidate for a low redshift quasar by Abraham et al (2012), who mainly characterised the nature of sources (quasars, galaxies, or stars) based on the colour-colour diagrams by using the SDSS7 data. The source is located in the region of quasars based on the colours of the SDSS7, but the u band magnitude of the SDSS9 survey is significantly different from that of SDSS7.…”

Five fake ultra-luminous X-ray sources

“…In SDSS DR9 the density of stars with magnitude r ≤ 16.58 at galactic latitudes >20 degrees is ∼430 per square degree, from which we obtained a probability for at least a chance alignment ∼2 × 10 −4 . Abraham et al (2012) used the SDSS photometric survey to estimate a density of QSOs at limiting magnitude g ∼ 20 of 18 per square degree. Consequently, the probability to have at least one chance alignment in the three cases considered is 1.3 × 10 −5 .…”

Survey of Object-Based Data Reduction Techniques in Observational Astronomy

“…More specialized random sampling strategies related to stratified sampling -preserving the distribution of objects among classes -were also identified in the literature of the subject. For example, study [1] concerning the classification of six million unresolved photometric detections from SDSS survey obtained training data by supplementing random sample with under-represented examples. With this approach a well-known issue with random sampling, namely: poor representation of the sparsely represented examples is being alleviated.…”