Detection of WHIM in the Planck data using Stack First approach

We detect the diffuse thermal Sunyaev-Zeldovich (tSZ) effect from the gas filaments between the Luminous Red Galaxy (LRG) pairs using a new approach relying on stacking the individual frequency maps. We apply and demonstrate our method on ~88000 LRG pairs in the SDSS DR12 catalogue selected with an improved selection criterion that ensures minimal contamination by the Galactic CO emission as well as the tSZ signal from the clusters of galaxies. We first stack the Planck channel maps and then perform the Internal Linear Combination method to extract the diffuse $y_{\rm sz}$ signal. Our $Stack$ $First$ approach makes the component separation a lot easier as the stacking greatly suppresses the noise and CMB contributions while the dust foreground becomes homogeneous in spectral-domain across the stacked patch. Thus one component, the CMB, is removed while the rest of the foregrounds are made simpler even before component separation algorithm is applied. We obtain the WHIM signal of $y_{\rm whim}=(3.78\pm 0.37)\times 10^{-8}$ in the gas filaments, accounting for the electron overdensity of $\sim 13$. We estimate the detection significance to be $\gtrsim 10.2\sigma$. This excess $y_{\rm sz}$ signal is tracing the warm-hot intergalactic medium and it could account for most of the missing baryons of the Universe. We show that the $Stack$ $First$ approach is more robust to systematics and produces a cleaner signal compared to the methods relying on stacking the $y$-maps to detect weak tSZ signal currently being used by the cosmology community.