Clusters of galaxies are useful tools to constrain cosmological parameters but only when we have a correct knowledge of all systematics contributing to total mass estimate. Currently, the studies of the CMB primary anisotropies and the clusters abundance derive inconsistent results. In fact, inaccurate inferred cluster masses could induce biased cosmological results, see as an example the wrong cluster number count. To alleviate this tension it is critical to estimate the cluster total mass having a good control of the mass bias induced by the difference between the true mass of clusters and the mass, inferred by using hydrostatic equilibrium (HE) approximation and/or the virial theorem and/or galaxies kinematics and/or background sources lensing effect. This is among the hot topics of current cosmological studies. Multi-probe observations of clusters, i.e. observations in the optical band of the galaxies members and of background objects lensing effect, and in X-ray and millimetre bands, by the Sunyaev-Zel'dovich effect (SZ), to explore the IntraCluster Medium (ICM), allow to infer and to calibrate the mass budget.
In this Research Project we plan to deeply investigate the amount and the radial distribution of matter inside clusters of galaxies, in terms of stellar, gas and Dark Matter components by using observations and hydrodynamical simulations. We have access to high angular resolution SZ maps of 50 clusters with the mm-camera NIKA2 at 30m IRAM telescope in Spain. For the same sample of clusters, we have optical maps with the on-going optical follow-up with TNG and X-ray maps of the same objects provided by XMM satellite. In support of these observations we have access to state-of-art hydrodynamical simulations, MUSIC and The Three Hundred. Lensing data will be available with EUCLID satellite while in the meantime with HSC-Subaru observations. We plan to investigate galaxy clusters even in the presence of generalized framework of modified gravity theories.
This Project is focused on the study of the mass of clusters of galaxies to strengthen the effectiveness of those objects as reliable cosmological tools. The fundamental contribution of this research is the multi-probe approach: i.e. multi-wavelength observations of clusters of galaxies in optical and X-ray bands and at millimetre wavelengths by SZ effect supported by state-of-the-art hydrodynamic simulations. The participants of this Project have full access to:
1) the most recent and accurate cosmological hydrodynamic simulations of a large volume (1Gpc box) with synthetic massive clusters in a large redshift range such as MUSIC and, the more recent one, The Three Hundred Project. Both datasets, starting from DM evolution, include baryons with different physical processes: i.e. radiative physics such as cooling star formation, UV photoionization, Supernovae feedbacks and Active Galactic Nuclei;
2) the ongoing high resolution and high sensitivity observations at millimetre wavelengths with the millimetre camera NIKA2 at IRAM 30m telescope of 50 clusters (SZ Large Program, 25 clusters already observed) and medium spectral resolution observations with the new spectrometer, KISS, installed at the focal plane of QUIJOTE telescope in Tenerife. We have priority access to all these observational data. Future observations of high redshifts sources with the camera MUSCAT and all sky survey data with the next EUCLID satellite will complement the available data;
3) the deep optical photometry follow-up at r'-, i'- and z'-bands of the NIKA2 clusters with the spectrometer DOLORES at Telescopio Nazionale Galileo.
We believe that the combination of such a kind of multi-wavelength observations with state-of-the-art numerical hydrodynamical simulations is a powerful approach to keep under control observational systematics and to validate the observations capabilities making a relevant advancement on the current knowledge of estimating the mass of clusters of galaxies.
We have already proven the power of this innovative combined approach, by generating a sample of synthetic clusters, the so called `NIKA2 twin sample¿, with characteristics (mass and redshift ranges) like the catalogue of the SZ LP of NIKA2. The first study of ICM pressure radial profiles, derived from simulated observations NIKA2-like, has highlighted the high angular resolution capabilities of this camera, able to detect the presence of ICM disturbances up to R500 (the cluster centred radius where the cluster density is 500 times the Universe critical density) even at high redshifts and their impact on the HSE approach, see Ruppin F. et al., A&A 631, A21 (2019).
Already 25 clusters, out of 50, have been observed with NIKA2 and for all of them X-ray follow-up with XMM-Newton satellite is almost completed while. The optical maps will be provided during next year. The combination of such a powerful multi-wavelength dataset will allow to study the baryons distribution in clusters even in a high redshift range.
In this way we have the possibility to shed light on the hot topic of the mass bias to reduce the current tension with between cosmological constraints from CMB and Planck cluster catalogue.
As far as concern gravitational lensing, the future EUCLID mission will give the possibility to measure shear and convergence fields with an unprecedented precision and accuracy. A strong effort on systematics control is crucial to reach this goal. At the same time, it is crucial to investigate the potentiality of new tools like flexions and higher order statistics to discriminate between different dark energy and DM scenarios. All these topics are at the frontier of the present knowledge because until now we hadn't enough high-quality data. In the meantime, we are planning to use Subaru HSC lensing data thanks to existing and future collaborations.