A comparison of CMB angular power spectrum estimators at large scales: the TT case

In the context of cosmic microwave background (CMB) data analysis, we compare the efficiency at large scale of two angular power spectrum algorithms, implementing, respectively, the quadratic maximum likelihood (QML) estimator and the pseudo-spectrum (pseudo-C-l) estimator. By exploiting 1000 realistic Monte Carlo simulations, we find that the QML approach is markedly superior in the range 2 < l < 100. At the largest angular scales, e.g. l < 10, the variance of the QML is almost 1/3 (1/2) that of the pseudo-C-l, when we consider the WMAP kq85 (kq85 enlarged by 8 degrees) mask, making the pseudo-spectrum estimator a very poor option. Even at multipoles 20 < l < 60, where pseudo-C-l methods are traditionally used to feed the CMB likelihood algorithms, we find an efficiency loss of about 20 per cent, when we considered the WMAP kq85 mask, and of about 15 per cent for the kq85 mask enlarged by 8 degrees. This should be taken into account when claiming accurate results based on pseudo-C-l methods. Some examples concerning typical large-scale estimators are provided.