Photo-z optimization for measurements of the BAO radial scale

Baryon Acoustic Oscillations (BAO) in the radial direction offer a method to directly measure the Universe expansion history, and to set limits to space curvature when combined to the angular BAO signal. In addition to spectroscopic surveys, radial BAO might be measured from accurate enough photometric redshifts obtained with narrow-band filters. We explore the requirements for a photometric survey using Luminous Red Galaxies (LRG) to competitively measure the radial BAO signal and discuss the possible systematic errors of this approach. Using the LRG spectra obtained from SDSS, we find that the optimal system consists of ∼30 filters of width Δλ/λ ∼ 0.02. A 20$"C generally necessary at the filters on the red side of the Balmer-break to reach the target photometric accuracy σ z = 0.003(1+z) for measuring radial BAO. The presence of outliers can significantly degrade this: in any practical application, being able to efficiently remove outliers may require ancillary observations. We estimate that a 5-year survey in a dedicated telescope with etendue in excess of 60 m 2deg 2 would be necessary to obtain a high enough density of galaxies to measure the radial BAO with shot noise smaller than cosmic variance up to z = 0.85. We conclude that spectroscopic surveys have a superior performance than photometric ones for measuring BAO in the radial direction. © 2009 IOP Publishing Ltd and SISSA.