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WISExSCOS Photometric Redshift Catalogue (WISExSCOSPZ)

The archive also hosts the WISExSCOS Photometric Redshift catalogue, constructed by cross-matching ALLWISE and the SuperCOSMOS all-sky samples and employing the artificial neural network approach (the ANNz algorithm, Collister & Lahav 2004), trained on the GAMA-II redshift survey (Driver et al. 2011; Liske et al. 2015). The derived photometric redshifts have errors nearly independent of distance, with an overall accuracy of sigmaz/(1+z) = 0.033, and a very small percentage of outliers. These redshift estimates have a typical precision of 15%. The resulting WISExSuperCOSMOS sample contains (after appropriate masking) about 18.5 million galaxies with a median redshift of z=0.2. This catalogue is described in Bilicki et al. 2016, ApJS, 225, 1, 5. The arXiv version can be found here. Please reference this paper and acknowledge the Wide Field Astronomy Unit (WFAU) when using these data.

The arrangement of the data is simple: all the data is in the wiseScosPhotoz table. The table is linked to the 2 input photometric tables through the the scosID, which links to the SSA Source objID and the wiseID, which links to the WISE allwise_sc cntr, when fromAllSky=0 and to WISE wise_allskysc cntr, when fromAllSky=1. It is much faster to match through the integer IDs rather than the designations. Matching is possible to previous releases of UKIDSS and VISTA and OSA ATLAS surveys through the existing neighbour tables to allwise_sc. Future releases of these surveys will be matched directly to the WISExSCOS Photometric Redshift Catalogue.

WISExSCOS uses SuperCOSMOS gCorMag magnitudes from the SSA Source table, but have applied some important corrections, as were done for the 2MPZ:
1) There is a small offset between the UKST plates in the Southern hemispere and the POSS-II plates in the Northern hemisphere (found by comparison with SDSS) that was corrected by changing the Southern hemisphere (δ1950<2.5):

  • BScorr=B+0.03(B-R)2-0.005(B-R)
  • RScorr=R+0.03(B-R)2-0.06(B-R)+0.015
  • 2) All of the magnitudes were corrected for Galactic extinction using the Schlegel et al. 1998, ApJ, 500, 525 dust maps, with revised coefficients derived from Schlafly & Finkbeiner 2011. In the case of SuperCOSMOS, the Aλ/E(B-V) coefficients were AB/EBV=3.44 and AR/EBV=2.23, whereas for WISE w?mpro magnitudes they were AW1/EBV=0.169 and AW2/EBV=0.130 (Indebetouw et al. 2005). Note that these coefficients were substantially revised with respect to those used for 2MPZ.

    What is released

      The released sample is the core WISE x SuperCOSMOS catalogue that meets the following criteria (see Bilicki et al. 2016 for details):
    1. sources are outside Galactic latitudes of |b|>10 deg, with a Bulge cutout up to |b|>17 deg by the Galactic Centre;
    2. source magnitudes are within B<21 (AB) and R<19.5 (AB) and 13.8<W1<17 (Vega), all extinction-corrected;
    3. Galactic extinction at source position is 0.25 (as obtained from the Schlegel et al. 1998 maps);
    4. sources are not flagged as star nor quasar candidates according to the Bilicki et al. 2016 colour cuts.
    5. We also provide a mask that can be used to remove areas of contamination not accounted for by the above cuts. The mask in Healpix fits format of NSide=256 is available at the following link: WISExSCOSmask.fits.gz. It is a binary mask, where 1 means useful area, and 0 stands for pixels to be masked out.
    6. This catalogue includes 20,416,142 sources.

    We also make available some of the sources which do not meet the criterion. Namely, we provide a list of 2 million objects rejected from the main sample as possible stellar blends, with however a high probability of being a galaxy. These sources fulfil the conditions 1.-3. above but their W1-W2 colour is between 0 and the position-dependent limit adopted for the main sample (see Sec.4.2 and Appendix of Bilicki et al. 2016 for details). These objects are located at Galactic latitudes of roughly 10<|b|<48. Adding these sources improves the completeness of the sample, but it degrades its purity; see Fig. 7 in Bilicki et al. 2016. THESE REJECTS SHOULD BE USED WITH CARE.

    WISE x SuperCOSMOS sources which do not meet the above criteria are available on request from Maciej Bilicki (bilicki[at]strw.leidenuniv.nl). Note however that they are often unreliable as far as their extragalactic character and photometric redshifts are concerned.

    The following link can be used to download the full catalogue, wiseScosPhotoz160708.csv.gz.

    The data are also held in a queryable database in the SSA. For more information concerning the dataset and how to explore it, please see the WISExSCOSPZ, in the SQL Cookbook. The schema for this dataset is available in the Schema Browser.

    Please contact Maciej Bilicki bilicki.at.strw.leidenuniv.nl for scientific questions, and mar.at.roe.ac.uk for technical questions.



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    WFAU, Institute for Astronomy,
    Royal Observatory, Blackford Hill
    Edinburgh, EH9 3HJ, UK

    mar@roe.ac.uk
    24/10/2016