The effect of photometric redshift uncertainties on galaxy clustering and baryonic acoustic oscillations

TL;DR

This paper investigates how small photometric redshift errors affect galaxy clustering and BAO measurements, proposing methods to mitigate their impact and forecast future survey precisions.

Contribution

It provides analytic models and simulation results quantifying the effects of photo-z errors on clustering and BAO, and introduces a template to extract unbiased cosmological information.

Findings

Photo-z errors suppress small-scale clustering and increase shot noise.

Photo-z errors reduce BAO smearing from non-linear RSD effects.

Photo-z errors induce scale dependence, reducing constraints on the Hubble parameter.

Abstract

In the upcoming era of high-precision galaxy surveys, it becomes necessary to understand the impact of redshift uncertainties on cosmological observables. In this paper we explore the effect of sub-percent photometric redshift errors (photo-$z$ errors) on galaxy clustering and baryonic acoustic oscillations (BAO). Using analytic expressions and results from $1\,000$ $N$-body simulations, we show how photo-$z$ errors modify the amplitude of moments of the 2D power spectrum, their variances, the amplitude of BAO, and the cosmological information in them. We find that: a) photo-$z$ errors suppress the clustering on small scales, increasing the relative importance of shot noise, and thus reducing the interval of scales available for BAO analyses; b) photo-$z$ errors decrease the smearing of BAO due to non-linear redshift-space distortions (RSD) by giving less weight to line-of-sight modes; and c) photo-$z$ errors (and small-scale RSD) induce a scale dependence on the information encoded in the BAO scale, and that reduces the constraining power on the Hubble parameter. Using these findings, we propose a template that extracts unbiased cosmological information from samples with photo-$z$ errors with respect to cases without them. Finally, we provide analytic expressions to forecast the precision in measuring the BAO scale, showing that spectro-photometric surveys will measure the expansion history of the Universe with a precision competitive to that of spectroscopic surveys.