The existence of Dark Matter (DM) in the Universe is inferred indirectly by its gravitational effect. Several observations of the motions of stars and gas in galaxies, cluster galaxy radial velocities, hot gas properties inside clusters of galaxies, and gravitational lensing of distant galaxies by foreground galaxy clusters suggest the presence of large amounts of DM. Currently we know that DM makes up 23% of the total mass-energy density of the Universe.
Among the different targets to trace and to infer the presence of DM, in this Project we focus on 1) clusters of galaxies, giving us some of the strongest clues as to the nature of this component of the Universe and 2) gravitational lensing and cosmic shear tomography, the most promising tools to both investigate the nature of DM and dark energy.
The observed radial velocities of cluster galaxies suggest dynamically based cluster masses that are factors of 10, or even more, higher than that deduced by adding up the observed cluster mass (stars, IntraCluster Medium (ICM) and dust) content. Similar inference derives from gravitational lensing approach.
In this Project we plan to deeply study the amount and radial distribution of DM inside clusters of galaxies. Indeed, the clusters total mass can be inferred by different observables and with different methods. The importance of this study is because inaccurate cluster masses could induce biased cosmological results, see as an example by possible wrong cluster number counts. Multi-probe observations of clusters (by X-ray emission, in optical bands and by Sunyaev-Zel¿dovich (SZ) effect) are useful to calibrate the mass budget and to better tuning the mass bias existing between the total mass (unknown) and the mass inferred by X-ray or SZ observations under the assumption of ICM satisfying hydrostatic equilibrium in the gravitational potential well.
This is among the hot topics of current cosmological studies.
This Project is focused on the study of DM distribution in the Universe and its evolution in redshift by 1) employing hydrodynamic simulations and observations of clusters of galaxies to strengthen the effectiveness of those objects as cosmological probes and 2) measuring shear and convergence fields with the incoming satellite EUCLID.
The participants of this Project have full access to
1) the more recent and accurate cosmological hydrodynamic simulations of 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 new camera NIKA2 at IRAM 30m telescope of 50 clusters (SZ Large Program) 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.
The combination of such a kind of observations with numerical hydrodynamical simulations is a powerful approach to keep under control observational systematics and to validate the observations capabilities.
We have already proven this innovative combined approach, generating a sample of synthetic clusters, the ¿twin sample¿, with characteristics (mass and redshift ranges) like the SZ LP of NIKA2. The first study of ICM pressure radial profiles, derived from simulated observations NIKA2-like, has shown the high angular resolution capabilities of this camera, able to detect the presence of ICM disturbances up to R500 (the radius where the cluster density is 500 times the critical density) even at high redshifts and their impact on the HSE approach, see Ruppin F. et al., 2019.
Already 19 clusters out of 50 have been observed with NIKA2 and for all of them X-ray and optical follow-ups with XMM-Newton satellite and Gran Telescopio Canarias are almost completed. The combination of such a powerful multi-wavelength dataset will allow to study the baryons, and inferring DM, 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 in order to reduce the current tension with between cosmological constraints from CMB and Planck cluster catalogue.
As far as concern gravitational lensing, EUCLID 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.
References
Ruppin F. et al. accepted on A&A (2019)