Extraction of photometric Quasars (QSOs) candidates

In this page, you will find a description of the method for the extraction of photometric QSOs candidates described in the paper "Quasar candidates selection in the Virtual Observatory era" from D'Abrusco et al. submitted to MNRAS (preprint).

download the catalogues of QSOs candidates extracted from the SDSS DR7 photometric survey


The inspiring principle of this work is the application of statistical and data-mining techniques to obtain a clustering of astronomical sources inside a photometric parameter space and fully characterize the distribution of different types of sources inside this parameter space. This concept has been applied to the problem of the selection of QSOs candidates from broadband photometric data by exploiting the availability of large spectroscopic bases of knowledge (BoK: i.e., samples of sources with a reliable classification).

The procedure for the extraction of candidates can be summarized as follows:

  • A BoK consisting of a sample of stellar sources with spectroscopic classification is clustered inside the colour parameter space. This BoK is drawn from the catalogue of photometric sources from where, at the end of the process, the new QSOs candidates will be extracted.
  • Several possible partitions of the distribution of sources of the BoK inside the colour space are produced by a combination of two clustering algorithm: PPS and NEC.
  • The members of each cluster of each different partition are labelled using the BoK classification.
  • Amongst all the possible partitions in the colour space, the one allowing the best separation between clusters populated mainly by confirmed QSOs ("successful" clusters) and clusters populated mainly by contaminants is considered.
  • The new candidates QSOs are selected as the photometric sources which are associated, in the colour space, to the "successful" clusters by a suitable distance definition.
The details of the method and algorithms can be found in the paper.

back to top page

Catalogue of QSOs

The catalogues in this page represent the application to the SDSS DR7 photometric data of the method for the extraction of photometric QSOs candidates discussed in the paper "Quasar candidates selection in the Virtual Observatory era" from D’Abrusco et al., submitted to MNRAS (preprint).
In order to ease the download of the massive dataset, the catalogue is split in distinct files each corresponding to a different "stripe", i.e. a patch of the sky defined (in the SDSS jargon) as "a line of constant eta, bounded on the north and south by the edges of the two strips that make up the stripe, and bounded on the east and west by lines of constant lambda. Because both strips and stripes are defined in "observed" space, they are rectangular areas which overlap as one approaches the poles."
The content of the catalogue files (FITS format) is described in the table below.

Column Name Description
1 catalogueID  Candidate ID
2 objID SDSS ID of the source
3 ra Right Ascension (J2000)
4 dec Declination (J2000)
5 psfMag_u PSF magnitude in u band
6 psfMag_q PSF magnitude in q band
7 psfMag_r PSF magnitude in r band
8 psfMag_i PSF magnitude in i band
9 psfMag_z PSF magnitude in z band
10 clusterID Cluster ID

Click on the name of the file and save it to your hard-drive. Enjoy!

stripe 9 stripe 27
stripe 10 stripe 28
stripe 11 stripe 29
stripe 12 stripe 30
stripe 13 stripe 31
stripe 14 stripe 32
stripe 15 stripe 33
stripe 16 stripe 34
stripe 17 stripe 35
stripe 18 stripe 36
stripe 19 stripe 37
stripe 20 stripe 38
stripe 21 stripe 39
stripe 22 stripe 42
stripe 23 stripe 43
stripe 24 stripe 44
stripe 25 stripe 76
stripe 26 stripe 82
  stripe 86

back to top page


Drawing of a full cube

Leonardo da Vinci

De Divina Proportione, Luca Pacioli, Milan, 1497

Publication Search Engines

[+] astro-ph


[+] IAU

[+] CDS Bibliographic


Scientific Media Resources
[+] ESO News

[+] ESA Press

[+] INAF Media

[+] Ateneapoli

[+] Caltech Media

[+] NASA News

[+] NRAO Press

[+] IVOA News

[+] CERN Press