We intend to observe a cosmologically representative, large sample of clusters with redshift between 0.5 and 1.5. The main output of the program will be the study of the redshift evolution of the cluster pressure profile as well as that of the scaling laws relating the cluster global properties, the Y (integrated Compton parameter) and T (temperature) for example, to their mass. This will be achieved by combining the NIKA2 data with ancillary data including X-rays and optical observations and will lead to significant improvements on the use of clusters of galaxies to draw cosmological constraints.

Sunyaev-Zel’dovich effect from clusters of galaxies

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 the incredibly high masses characterizing this kind of structures (1013 – 1015 MSun). Due to its physical state, the ICM is responsible of a secondary anisotropy of the Cosmic Microwave Background (CMB), which is the thermal Sunyaev-Zel’dovich (SZ) effect. Through their path toward us, CMB photons interact with free hot electrons in the ICM. After this interaction a fraction of CMB photons is moved to higher energies, with a resulting flux decrement (increment) at frequencies below (above) 217 GHz. The amplitude of the deformation is proportional to the integral of the pressure of the electron population along the line of sight. While optical and X-ray cluster signals are affected by cosmological dimming, this is not the case for the tSZ cluster signal. Therefore the tSZ effect allows us to detect and study clusters of galaxies at higher redshifts, where their number and distribution is the most sensitive to the underlying cosmology.

Since the ICM is a very good tracer of the dark matter distribution, ICM observables (like the SZ signal) can provide a valuable tool for cosmological investigation with clusters, as long as we are able to convert them into mass estimates. From baryonic observables the total mass can be inferred through scaling relations, which correspond to power laws obtained in a simplified scenario in which gravity is the only process driving cluster evolution. At present, the systematic uncertainties affecting the observable to mass relations represent the limit for cluster-derived cosmological constraints.

NIKA2 for SZ and cosmology with clusters

In the last few years, tSZ-selected cluster catalogues containing hundreds of candidates have finally been produced, with arcmin resolution, by the South Pole Telescope (SPT, Reichardt et al. 2013), the Atacama Cosmology Telescope (ACT, Hasselfield & ACT Collaboration 2013), the Planck Satellite (Planck Collaboration 2013) and APEX-SZ (Bender et al. 2014). tSZ surveys have proved to be competitive and complementary with respect to traditional methods of cluster detection (e.g. X-ray, optical). Observations through the tSZ effect are now providing a reliable tool to push cluster detection and characterization to higher redshifts

The use of tSZ- selected cluster samples for cosmological purposes requires the understanding of how matter is distributed and the evaluation of the scatter that disturbed systems may introduce in the tSZ integrate flux (Y) to total cluster mass (M) relation. The Planck, ACT, SPT and APEX-SZ resolutions only allow detailed study of the ICM morphology only for low redshift clusters (z < 0.2). Therefore, high angular resolution measurements of cluster pressure profiles are a mandatory step for precise cluster cosmology. Indeed, we still need to investigate how cluster properties evolve with time (and so with redshift) in order to understand the mechanisms ruling the formation and the evolution of structure. Moreover, pressure profiles Pe(r) with spatial resolution comparable to those of X-ray derived electron densities (ne(r)) can also be used to study the cluster radial distribution of temperature (Te(r) ∝ Pe(r)/ne(r)) and entropy (K(r) ∝ Pe(r)/ne(r)5/3), which are essential to unveil the cluster thermodynamic history.

The NIKA2 camera is particularly well adapted for high-resolution observations of the tSZ effect from clusters of galaxies : it operates simultaneously at two frequency bands, 150 and 260 GHz, where the distortion of the CMB spectrum due to the thermal SZ cluster signal is negative and positive, respectively. NIKA2 is made of arrays of thousands of high sensitive Kinetic Inductance Detectors (KIDs). In particular we expect sensitivity in Compton parameter units of 4x10-5 per hour and per beam. This should allow us to obtain reliable tSZ detections of clusters of galaxies in few hours. NIKA2 coupled to the IRAM 30 m telescope should allow us to map clusters of galaxies to a resolution of typically 12 to 20 arcsec within a 6.5 arcmin diameter FOV. This is well adapted for medium and high redshift clusters for which we expect typical angular sizes of about 6-11 arcmin.

The interest of NIKA2 for tSZ observations of clusters of galaxies has already been partially demonstrated by the results of NIKA, which is a prototype instrument operating at the IRAM 30 m telescope. In Adam et al. 2013 we report the first SZ observation ever performed with a KID-based instrument.

SZ target selection for the NIKA2 large program

We intend to observe a large (≥ 50) representative sample of clusters of galaxies with 0.5 < z < 1.5, in order to study of the calibration of the SZ flux as a mass proxy, its evolution with redshift and cluster dynamics, as well as the redshift evolution of the universal cluster pressure profile and the deviations from its mean behavior, due to cluster complex astrophysics and thermodynamical history. The natural basis for selecting the sources list will be clusters from already existing tSZ based cluster samples, for which we already have estimates of the total tSZ flux. In particular we are thinking about clusters observable in the North hemisphere from the Planck and ACT samples. In order to build a cosmological representative catalogue we will select a homogeneous distribution in redshift, in order to separate the cluster sample in several redshift bins, and both relaxed and dynamically disturbed systems, to ensure representativeness of the different morphologies characterizing a realistic cluster population.

Mis à jour le 28 septembre 2023