Galaxy tagging: Photometric redshift refinement and group richness enhancement

We present a new scheme, GALTAG, for refining the photometric redshift measurements of faint galaxies by probabilistically tagging them to observed galaxy groups constructed from a brighter,magnitude-limited spectroscopy survey. First, thismethod is tested on the DESI lightcone data constructed on the GALFORM galaxy formation model to tests its validity. We then apply it to the photometric observations of galaxies in the Kilo-Degree Imaging Survey (KiDS) over a 1 deg² region centred at 15^h. This region contains Galaxy and Mass Assembly (GAMA) deep spectroscopic observations (i band<22) and an accompanying group catalogue to r band < 19.8. We demonstrate that even with some trade-off in sample size, an order of magnitude improvement on the accuracy of photometric redshifts is achievable when using GALTAG. This approach provides both refined photometric redshift measurements and group richness enhancement. In combination these products will hugely improve the scientific potential of both photometric and spectroscopic data sets. The GALTAG software will be made publicly available at https://github.com/pkaf/galtag.git.