SZ effect or Not? - Detecting most galaxy clusters' main foreground effect

TL;DR

This paper introduces a new model-independent method to analyze the impact of galaxy clusters on CMB data, revealing that most clusters may not produce the expected thermal SZ effect, challenging previous assumptions.

Contribution

The study presents a novel, model-independent approach to confirm the SZ effect in galaxy clusters using WMAP data, highlighting unexpected results for optically selected samples.

Findings

Planck ESZ and X-ray clusters show temperature depression consistent with SZ effect.

Optically selected clusters unexpectedly increase CMB temperature, contradicting SZ predictions.

Main foreground effect of most clusters may not be the thermal SZ effect.

Abstract

Galaxy clusters are the most massive objects in the Universe and comprise a high-temperature intracluster medium of about 10^7 K, believed to offer a main foreground effect for cosmic microwave background (CMB) data in the form of the thermal Sunyaev-Zel'dovich (SZ) effect. This assumption has been confirmed by SZ signal detection in hundreds of clusters but, in comparison with the huge numbers of clusters within optically selected samples from Sloan Digital Sky Survey (SDSS) data, this only accounts for a few per cent of clusters. Here we introduce a model-independent new method to confirm the assumption that most galaxy clusters can offer the thermal SZ signal as their main foreground effect. For the Wilkinson Microwave Anisotropy Probe (WMAP) seven-year data (and a given galaxy cluster sample), we introduced a parameter d1 as the nearest-neighbour cluster angular distance of each pixel, then we classified data pixels as 'to be' (d1--> 0 case) or 'not to be' (d1 large enough) affected by the sample clusters. By comparing the statistical results of these two kinds of pixels, we can see how the sample clusters affect the CMB data directly. We find that the Planck Early Sunyaev-Zel'dovich (ESZ) sample and X-ray samples (~10^2 clusters) can lead to obvious temperature depression in the WMAP seven-year data, which confirms the SZ effect prediction. However, each optically selected sample (>10^4 clusters) shows an opposite result: the mean temperature rises to about 10uK. This unexpected qualitative scenario implies that the main foreground effect of most clusters is $not$ always the expected SZ effect. This may be the reason why the SZ signal detection result is lower than expected from the model.