In Quest of Devising Tools of Probing Cosmology and Gravity using Galaxy Clusters

Abstract

Galaxy clusters are the biggest gravitationally bound objects in the Universe with various properties which allow us to test gravitational and cosmological models. One such way of testing theoretical models is by directly measuring density profiles of different matter components (i.e. weak lensing and X-ray provide information about baryon and dark matter mass distributions). A recent version of Emergent Gravity (EG) predicts a specific connection between baryonic and dark matter which can be directly tested using galaxy clusters. By using a sample of 23 galaxy clusters, we find that the EG predictions (based on no dark matter) are acceptable fits only near the virial radius. In the cores and in the outskirts, the mass profile shape differences allow us to rule out EG at >5 sigma level. However, when we account for systematic uncertainties in the observed weak-lensing and X-ray profiles, we conclude that we cannot formally rule our EG as an alternative to dark matter on the cluster scale and that we require better constraints on the weak-lensing and gas mass profile shapes in the region 0.3< r/r200< 1. We also show that EG itself allows flexibility in its predictions, which can allow for good agreement between the observations and the predictions. The second way, which is address in current manuscript, to probe theory is based on escape velocity profiles of galaxy clusters, which has been shown to be a competitive probe of cosmology in an accelerating universe. Projection onto the sky is a dominant systematic uncertainty for statistical inference, since line-of-sight projection of the galaxy positions and velocities can suppress the underlying 3D escape-velocity edge. In this work, we develop the approach based on idea of creating N galaxies with positions and velocities on Keplerian orbits, given richness and the line-of-sight velocity dispersion. We then compare the analytical escape edge to those from N-body simulations. We show that given high enough sampling, the 3D escape velocity edge is in fact observable without systematic bias or suppression with < 1% accuracy over the range 0< r/r200< 1. We show that the approach model the amount of the edge suppression with ~2% accuracy and ~5% precision for massive (>10^14 Solar masses) systems over the range 0.4< r/r200< 1. We show that the numerically modeled suppression is independent of velocity anisotropy over the range (-2.5,0.5). Finally, we show that suppression is mass and cosmology independent and can be successfully modeled by inverse power-law (Zv=1+(N0/N)^L) with best-fit parameters N0=14.205, L=0.467. We conclude that the 3D cluster escape velocity profile can be inferred from projected phase-space data without knowledge of cosmology or the use of simulations. We applied this suppression function to test cosmology and our preliminary results produced a tight constraints on cosmological parameters. By statistically analyzing the set of 38 galaxy clusters, we were able to constraint matter energy-density Omega_m=0.325 (+0.014(stat)+0.003(sys), -0.021(stat)-0.001(sys)) and the Hubble constant h=0.733 (+0.007(stat)+0.035(sys), -0.006(stat)-0.029(sys)) in the framework of flat universe and fixed equation of state of dark energy (w=-1). The systematic error budget includes 5% uncertainties on the weak lensing mass calibration and 5% uncertainties in the density model differences between the NFW and the Einasto functions. This result is in a good agreement with other probes, while in general favor CMB observations of Omega_m by Planck Collaboration and h by using Cepheids.