The ISW-tSZ cross correlation: ISW extraction out of pure CMB data

TL;DR

This paper proposes a method to detect the ISW-tSZ cross correlation directly from CMB data using frequency dependence, enabling dark energy studies without external large-scale structure data.

Contribution

It introduces a novel approach to extract the ISW-tSZ cross correlation from multi-frequency CMB data alone, avoiding reliance on external tracers.

Findings

Masking low redshift clusters improves signal-to-noise ratio.

Foreground residuals must be controlled below ~0.04 μK^2 at l=10.

Method can independently probe Dark Energy and density perturbations.

Abstract

If Dark Energy introduces an acceleration in the universal expansion then large scale gravitational potential wells should be shrinking, causing a blueshift in the CMB photons that cross such structures (Integrated Sachs-Wolfe effect, [ISW]). Galaxy clusters are known to probe those potential wells. In these objects, CMB photons also experience inverse Compton scattering off the hot electrons of the intra-cluster medium, and this results in a distortion with a characteristic spectral signature of the CMB spectrum (the so-called thermal Sunyaev-Zel'dovich effect, [tSZ]). Since both the ISW and the tSZ effects take place in the same potential wells, they must be spatially correlated. We present how this cross ISW-tSZ signal can be detected in a CMB-data contained way by using the frequency dependence of the tSZ effect in multi frequency CMB experiments like {\it Planck}, {\em without} requiring the use of external large scale structure tracers data. We find that by masking low redshift clusters, the shot noise level decreases significantly, boosting the signal to noise ratio of the ISW--tSZ cross correlation. We also find that galactic and extragalactic dust residuals must be kept at or below the level of ~0.04 muK^2 at l=10, a limit that is a factor of a few below {\it Planck}'s expectations for foreground subtraction. If this is achieved, CMB observations of the ISW-tSZ cross correlation should also provide an independent probe for the existence of Dark Energy and the amplitude of density perturbations.