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Bilicki, M.,Hoekstra, H.,Brown, M. J. I.,Amaro, V.,Blake, C.,Cavuoti, S.,de Jong, J. T. A.,Georgiou, C.,Hildebrandt, H.,Wolf, C.,Amon, A.,Brescia, M.,Brough, S.,Costa-Duarte, M. V.,Erben, T.,Glazebroo EDP Sciences 2018 Astronomy and astrophysics Vol.616 No.-
<P>We present a machine-learning photometric redshift (ML photo-<I>z</I>) analysis of the Kilo-Degree Survey Data Release 3 (KiDS DR3), using two neural-network based techniques: ANNz2 and MLPQNA. Despite limited coverage of spectroscopic training sets, these ML codes provide photo-<I>z</I>s of quality comparable to, if not better than, those from the Bayesian Photometric Redshift (BPZ) code, at least up to <I>z</I>phot ≲ 0.9 and <I>r</I> ≲ 23.5. At the bright end of <I>r</I> ≲ 20, where very complete spectroscopic data overlapping with KiDS are available, the performance of the ML photo-<I>z</I>s clearly surpasses that of BPZ, currently the primary photo-<I>z</I> method for KiDS. Using the Galaxy And Mass Assembly (GAMA) spectroscopic survey as calibration, we furthermore study how photo-<I>z</I>s improve for bright sources when photometric parameters additional to magnitudes are included in the photo-<I>z</I> derivation, as well as when VIKING and WISE infrared (IR) bands are added. While the fiducial four-band <I>ugri</I> setup gives a photo-<I>z</I> bias 〈<I>δz</I>/(1 + <I>z</I>)〉 = −2 × 10<SUP>−4</SUP> and scatter <I>σδz/(1+z)</I> < 0.022 at mean 〈<I>z</I>〉 = 0.23, combining magnitudes, colours, and galaxy sizes reduces the scatter by ~7% and the bias by an order of magnitude. Once the <I>ugri</I> and IR magnitudes are joined into 12-band photometry spanning up to 12 <I>μ</I>m, the scatter decreases by more than 10% over the fiducial case. Finally, using the 12 bands together with optical colours and linear sizes gives 〈<I>δz</I>/(1 + <I>z</I>)〉 < 4 × 10<SUP>−5</SUP> and <I>σ</I><I>δz</I>/(1+<I>z</I>) < 0.019. This paper also serves as a reference for two public photo-<I>z</I> catalogues accompanying KiDS DR3, both obtained using the ANNz2 code. The first one, of general purpose, includes all the 39 million KiDS sources with four-band <I>ugri</I> measurements in DR3. The second dataset, optimised for low-redshift studies such as galaxy-galaxy lensing, is limited to <I>r</I> ≲ 20, and provides photo-<I>z</I>s of much better quality than in the full-depth case thanks to incorporating optical magnitudes, colours, and sizes in the GAMA-calibrated photo-<I>z</I> derivation.</P>
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