Submegaparsec individual photometric redshift estimation from cosmic web constraints

We present a method, PhotoWeb, for estimating photometric redshifts of <it>individual galaxies</it>, and their equivalent distance, with megaparsec and even submegaparsec accuracy using the cosmic web as a constraint over photo-<it>z</it> estimates. PhotoWeb redshift errors of individual galaxies are of the order of Δ<it>z</it> ≃ 0.0007, compared to errors of Δ<it>z</it> ≃ 0.02 for current photo-<it>z</it> techniques. The mean redshift error is of the order of Δ<it>z</it> ≃ 5 × 10−5–5 × 10−4 compared to mean errors in the range Δ<it>z</it> ≃ 0.001–0.01 for the best available photo-<it>z</it> estimates in the literature. Current photo-<it>z</it> techniques produce redshift estimates with large errors due to the poor constraining power the galaxy's spectral energy distribution and projected clustering can provide. The cosmic web, on the other hand, provides the strongest constraints on the position of galaxies. The network of walls, filaments and voids occupy ∼ 10 per centof the volume of the Universe, yet they contain ∼ 95 per centof galaxies. The cosmic web, being a cellular system with well-defined boundaries, defines a restricted set of intermittent positions a galaxy can occupy along a given line of sight. Using the information in the density field computed from spectroscopic redshifts, we can narrow the possible locations of a given galaxy along the line of sight from a single broad probability distribution (from photo-<it>z</it>) to one or a few narrow peaks. Our first results improve previous photo-<it>z</it> errors by more than one order of magnitude allowing submegaparsec errors in some cases. Such accurate estimates for tens of millions of galaxies will allow unprecedented galaxy-Large Scale Structure (LSS) studies. In this work, we apply our technique to the Sloan Digital Sky Survey photo-<it>z</it> galaxy sample and discuss its performance and future improvements.