The distributions are very similar to the overall distribution of the unsampled cells of galaxy color space identified in M15, indicating that we are targeting the correct sources.

SOM-based photo-z performance
-----------------------------

The self-organizing map colored by the median photo-z of sources per cell (the left panel of Figure \[figure:colors\]) effectively defines a photometric redshift estimate for each galaxy based on its position in the *ugrizYJH* color space of *Euclid*/*WFIRST*. Figure \[figure:specz\_somz\] compares our Q=4 spectroscopic redshifts with the redshift that would be inferred based on the SOM, with encouraging results. The normalized median absolute deviation (a dispersion measure which is not sensitive to catastrophic outliers [@Ilbert09; @Dahlen13]) defined as $$\sigma_{\mathrm{NMAD}} = 1.48\times \mathrm{median}\Big(\frac{\lvert z_{p}-z_{s} \rvert}{1+z_{s}}\Big)$$ is 0.027 (2.7%) for the sample, which is quite low.

Using the standard definition of catastrophic photo-z outliers as those with $\lvert z_{p} - z_{s} \rvert/(1+z_{s}) \geq 0.15$, we measure a low outlier fraction of 3.8%. The measured bias, defined as $$\mathrm{mean}\Big(\frac{ z_{p}-z_{s}}{1+z_{s}}\Big)$$ is $\lesssim$0.1% after removing the catastrophic outliers. Further improvements to these results will result from folding in all spectroscopic information from C3R2 and other surveys to the  relation encoded by the SOM. Notably, these results are already competitive with or better 