Redshift-space distortions from the cross-correlation of photometric populations

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Several papers have recently highlighted the possibility of measuring redshift-space distortions from angular auto-correlations of galaxies in photometric redshift bins. In this work, we extend this idea to include as observables the cross-correlations between redshift bins, as an additional way of measuring radial information. We show that this extra information allows us to reduce the recovered error in the growth rate index γ by a factor of ∼2. Although the final error in γ depends on the bias and the mean photometric accuracy of the galaxy sample, the improvement from adding cross-correlations is robust in different settings. Another factor of 2–3 improvement in the determination of γ can be achieved by considering two galaxy populations over the same photometric sky area but with different biases. This additional gain is shown to be much larger than the one from the same populations when observed over different areas of the sky (with twice the combined area). The total improvement of ∼5 implies that a photometric survey such as Dark Energy Survey should be able to recover γ at the 5–10 per cent from the angular clustering in linear scales of two different tracers. It can also constrain the evolution of f(z) × σ8(z) in few bins beyond z ∼ 0.8–0.9 at the 10–15 per cent level per bin, compatible with recent constrains from lower z spectroscopic surveys. We also show how further improvement can be achieved by reducing the photometric redshift error.