Colour tomography

Lensing tomography with multi-colour imaging surveys can probe dark energy and the cosmological power spectrum. However, accurate photometric redshifts for tomography out to high redshift require imaging in five or more bands, which is expensive to carry out over thousands of square degrees. Since lensing makes coarse, statistical use of redshift information, we explore the prospects for tomography using limited colour information from two- or three-band imaging. With an appropriate calibration sample, we find that it is feasible to create up to four redshift bins using imaging data in just the g, r and i bands. We construct such redshift sub-samples from mock catalogues by clustering galaxies in colour space and discarding regions with poorly defined redshift distributions. The loss of galaxy number density decreases the accuracy of lensing measurements, but even losing half or more of the galaxies is not a severe loss for large area surveys. We estimate the errors on lensing power spectra and dark energy parameters with colour tomography and discuss trade-offs in survey area and filter choice. We discuss the systematic errors that may change our conclusions, especially the information needed to tackle intrinsic alignments.