Local non-Gaussianity causes correlations between large-scale perturbation modes and the small-scale power. The large-scale CMB signal has contributions from the integrated Sachs-Wolfe (ISW) effect, which does not correlate with the small-scale power. If this ISW contribution can be removed, the sensitivity to local non-Gaussianity is improved. Gravitational lensing and galaxy counts can be used to trace the ISW contribution; in particular, we show that the CMB lensing potential is highly correlated with the ISW signal. We construct a nearly optimal estimator for the local non-Gaussianity parameter $f_{\mathrm{NL}}$ and investigate to what extent we can use this to decrease the variance on $f_{\mathrm{NL}}$. We show that the variance can be decreased by up to 20% at Planck sensitivity using galaxy counts. CMB lensing is a good bias-independent ISW tracer for future more sensitive observations, though the fractional decrease in variance is small if good polarization data are also available.

• Received 10 September 2010

DOI:https://doi.org/10.1103/PhysRevD.83.023507