Ensemble photometric redshifts
  Upcoming imaging surveys, such as LSST, will provide an unprecedented view of
the Universe, but with limited resolution along the line-of-sight. Common ways
to increase resolution in the third dimension, and reduce misclassifications,
include observing a wider wavelength range and/or combining the broad-band
imaging with higher spectral resolution data. The challenge with these
approaches is matching the depth of these ancillary data with the original
imaging survey. However, while a full 3D map is required for some science,
there are many situations where only the statistical distribution of objects
(dN/dz) in the line-of-sight direction is needed. In such situations, there is
no need to measure the fluxes of individual objects in all of the surveys.
Rather a stacking procedure can be used to perform an `ensemble photo-z'. We
show how a shallow, higher spectral resolution survey can be used to measure
dN/dz for stacks of galaxies which coincide in a deeper, lower resolution
survey. The galaxies in the deeper survey do not even need to appear
individually in the shallow survey. We give a toy model example to illustrate
tradeoffs and considerations for applying this method. This approach will allow
deep imaging surveys to leverage the high resolution of spectroscopic and
narrow/medium band surveys underway, even when the latter do not have the same
reach to high redshift.
