Phenotypic redshifts with self-organizing maps: A novel method to characterize redshift distributions of source galaxies for weak lensing. (arXiv:1901.05005v1 [astro-ph.CO])

Phenotypic redshifts with self-organizing maps: A novel method to characterize redshift distributions of source galaxies for weak lensing. (arXiv:1901.05005v1 [astro-ph.CO])
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Wide-field imaging surveys such as the Dark Energy Survey (DES) rely on
coarse measurements of spectral energy distributions in a few filters to
estimate the redshift distribution of source galaxies. In this regime, cosmic
variance, shot noise, and selection effects limit the attainable accuracy of
redshift calibration and thus of cosmological constraints. We present a new
method to combine wide-field, few-filter measurements with catalogs from deep
fields with additional filters and sufficiently low photometric noise to break
degeneracies in photometric redshifts. The multi-band deep field is used as an
intermediary between wide-field observations and accurate redshifts, greatly
reducing cosmic variance, shot noise, and selection effects. Our implementation
of the method uses self-organizing maps to group galaxies into
textit{phenotypes} based on their observed fluxes, and is tested using a mock
DES catalog created from $N$-body simulations. It yields a typical uncertainty
on the mean redshift in each of five tomographic bins for a simulation of the
DES Year 3 weak-lensing tomographic analysis of $sigma_{Delta z} = 0.007$,
which is a 60% improvement compared to the Year 1 analysis. Although the
implementation of the method is tailored to DES, its formalism can be applied
to other large photometric surveys with a similar observing strategy.

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