Photometric redshifts for the Kilo-Degree Survey Machine-learning analysis with artificial neural networks

Journal article

We present a machine-learning photometric redshift (ML photo-z) 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-zs of quality comparable to, if not better than, those from the Bayesian Photometric Redshift (BPZ) code, at least up to zphot ≲ 0.9 and r ≲ 23.5. At the bright end of r ≲ 20, where very complete spectroscopic data overlapping with KiDS are available, the performance of the ML photo-zs clearly surpasses that of BPZ, currently the primary photo-z method for KiDS. Using the Galaxy And Mass Assembly (GAMA) spectroscopic survey as calibration, we furthermore study how photo-zs improve for bright sources when photometric parameters additional to magnitudes are included in the photo-z derivation, as well as when VIKING and WISE infrared (IR) bands are added. While the fiducial four-band ugri setup gives a photo-z bias 〈δz=(1 + z)〉 = -2 X 10^-4 and scatter σδz=(1+z) < 0.022 at mean 〈z〉 = 0.23, combining magnitudes, colours, and galaxy sizes reduces the scatter by ~7% and the bias by an order of magnitude. Once the ugri and IR magnitudes are joined into 12-band photometry spanning up to 12 μm, the scatter decreases by more than 10% over the fiducial case. Finally, using the 12 bands together with optical colours and linear sizes gives 〈δz=(1 + z)〉 < 4 X 10^-5 and σδz=(1+z) < 0.019. This paper also serves as a reference for two public photo-z 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 ugri measurements in DR3. The second dataset, optimised for low-redshift studies such as galaxy-galaxy lensing, is limited to r ≲ 20, and provides photo-zs of much better quality than in the full-depth case thanks to incorporating optical magnitudes, colours, and sizes in the GAMA-calibrated photo-z derivation.

Authors: Bilicki, M; Hoekstra, H; Brown, M; Amaro, V; Blake, C

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: EDP Sciences
• Journal: Astronomy and Astrophysics
• Volume: 616
• Pages: A69
• Publication date: 2018-08-21
• Acceptance date: 2018-04-30
• DOI: 10.1051/0004-6361/201731942
• EISSN: 1432-0746
• ISSN: 0004-6361
• Keywords: methods: numerical; catalogs; methods: statistical; methods: data analysis; large-scale structure of Universe; galaxies: distances and redshifts