Galaxy Tagging: photometric redshift refinement and group richness
  enhancement
  We present a new scheme, $\it{galtag}$, 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, this method is tested on the DESI light-cone 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 deg$^2$ region centred at 15$^\mathrm{h}$. 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 $\it{galtag}$. 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 datasets. The
$\it{galtag}$ software will be made publicly available at
https://github.com/pkaf/galtag.git.
