Galaxy clusters

Category sciences

Galaxy clusters using the Sunyaev-Zel’dovich effect

Clusters are mainly made up of dark matter (85%), while most of the baryons are present as a diffuse gas, the Intra-Cluster Medium (ICM), which is hot (106– 108 K) and completely ionized. Due to its physical state, the ICM is responsible of a secondary anisotropy of the Cosmic Microwave Background (CMB), which is the Sunyaev-Zel’dovich (SZ) effect (Sunyaev & Zeldovich 1972). SZ effect provides a way to probe the ICM gas bound in clusters without suffering from cosmological dimming. Its observation is therefore independent of redshift as long as the angular resolution of the observations is sufficient to resolve the clusters.

One can distinguish between thermal SZ effect for which the cosmic microwave background (CMB) photons are spectrally distorted by the electronic thermal pressure of the ICM, and kinetic SZ effect (kSZ) arising from the CMB doppler shift induced by the bulk motion of the cluster electrons (see the figure). While the tSZ is related to the integrated pressure along the line-of-sight and can be used as a mass proxy in cosmological studies, the kSZ is sensitive to the integrated line-of-sight electron density and the gas velocity with respect to the CMB reference frame. The kSZ signal can be an invaluable probe of the large scale bulk flows determined from galaxy cluster peculiar velocities. Measurement of the gas velocity distribution in galaxy clusters also provides insight into the physics of mergers. For the tSZ, mapping the relativistic corrections allows to determine the galaxy cluster temperature profile via the tSZ effect only, and thus independently of X-ray observations.

Constraints on the kSZ and the tSZ amplitudes toward one of the sub-cluster region of MACS J0717.5+3745. The black data points give the average brightness measured by NIKA. The shaded areas give the 68% confidence level constraints on the kSZ (red), the tSZ (blue) and the sum of the two (gray). Point source contamination has been removed using Herschel/SPIRE and NIKA observations. Figure from Adam et al. (2017).

The kSZ signal is subdominant to the tSZ unless the gas velocity reaches a few tenths of a percent of the speed of light. In addition, the spectral dependence of the kSZ is the same as that of the CMB. Direct observation of the kSZ signal within clusters is therefore particularly challenging.

Thanks to its spectroscopic capabilities, CONCERTO will enable an accurate separation of the tSZ, kSZ and relativistic corrections, as well as the extraction of the SZ signals from the other astrophysical components, as the cosmic infrared background and Galactic dust. The component separation will be the limiting factor for current and planned galaxy cluster surveys. CONCERTO field-of-view is very well adapted for medium and high redshift clusters for which we expect typical angular sizes of <20 arcmin.