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The Integrated Sachs-Wolfe (ISW) effect is a large-angle modulation of thecosmic microwave background (CMB), generated when CMB photons traverse evolvingpotential wells associated with large scale structure (LSS). Recent effortshave been made to reconstruct maps of the ISW signal using information fromsurveys of galaxies and other LSS tracers, but investigation into how surveysystematics affect their reliability has so far been limited. Using simulatedISW and LSS maps, we study the impact of galaxy survey properties andsystematic errors on the accuracy of reconstructed ISW signal. We find thatsystematics that affect the observed distribution of galaxies along the line ofsight, such as photo-z and bias-evolution related errors, have a relativelyminor impact on reconstruction quality. In contrast, however, we find thatdirection-dependent calibration errors can be very harmful. Specifically, wefind that in order to avoid significant degradation of our reconstructionquality statistics, direction-dependent number density fluctuations due tosystematics must be controlled so that their variance is smaller than $10^{-6}$(which corresponds to a 0.1% calibration). Additionally, we explore theimplications of our results for attempts to use reconstructed ISW maps to shedlight on the origin of large-angle CMB alignments. We find that there is only aweak correlation between the true and reconstructed angular momentumdispersion, which quantifies alignment, even for reconstructed ISW maps whichare fairly accurate overall.

Peeling off the late Universe: Reconstructing the ISW map with galaxy surveys