Hierarchical modeling and statistical calibration for photometric
  redshifts
  The cosmological exploitation of modern photometric galaxy surveys requires
both accurate (unbiased) and precise (narrow) redshift probability
distributions derived from broadband photometry. Existing methodologies do not
meet those requirements. Standard template fitting delivers interpretable
models and errors, but lacks flexibility to learn inaccuracies in the observed
photometry or the spectral templates. Machine learning addresses those issues,
but requires representative training data, and the resulting models and
uncertainties cannot be interpreted in the context of a physical model or
outside of the training data. We present a hierarchical modeling approach
simultaneously addressing the issues of flexibility, interpretability, and
generalization. It combines template fitting with flexible (machine
learning-like) models to correct the spectral templates, model their redshift
distributions, and recalibrate the photometric observations. By optimizing the
full posterior distribution of the model and solving for its (thousands of)
parameters, one can perform a global statistical calibration of the data and
the SED model. We apply this approach to the public Dark Energy Survey Science
Verification data, and show that it provides more accurate and compact redshift
posterior distributions than existing methods, as well as insights into
residual photometric and SED systematics. The model is causal, makes
predictions for future data (e.g., additional photometric bandpasses), and its
internal parameters and components are interpretable. This approach does not
formally require the training data to be complete or representative; in
principle it can even work in regimes in which few or no spectroscopic
redshifts are available.
