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Register a new userDark matter distribution in X-ray luminous galaxy clusters with Emergent Gravity
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We present the radial distribution of the dark matter in two massive, X-ray luminous galaxy clusters, Abell~2142 and Abell~2319, and compare it with the quantity predicted as apparent manifestation of the baryonic mass in the context of the Emergent Gravity scenario, recently suggested from Verlinde (2016). Thanks to the observational strategy of the xmm Cluster Outskirt Programme (X-COP), using the X-ray emission mapped with xmm and the SZ signal in the Planck survey, we recover the gas density, temperature and thermal pressure profiles up to $sim R_{200}$, allowing to constrain at unprecedented level the total mass through the hydrostatic equilibrium equation. We show that, also including systematic uncertainties related to the X-ray based mass modelling, the apparent dark matter shows a radial profile that has a shape different from the traditional dark matter distribution, with larger discrepancies (by a factor 2--3) in the inner ($r<200$ kpc) clusters regions and a remarkable agreement only across $R_{500}$.
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(Abriged) Assuming that the hydrostatic equilibrium holds between the intracluster medium and the gravitational potential, we constrain the NFW profiles in a sample of 44 X-ray luminous galaxy clusters observed with XMM-Newton in the redshift range 0.1-0.3. We evaluate several systematic uncertainties that affect our reconstruction of the X-ray masses. We measure the concentration c200, the dark mass M200 and the gas mass fraction within R500 in all the objects of our sample, providing the largest dataset of mass parameters for galaxy clusters in this redshift range. We confirm that a tight correlation between c200 and M200 is present and in good agreement with the predictions from numerical simulations and previous observations. When we consider a subsample of relaxed clusters that host a Low-Entropy-Core (LEC), we measure a flatter c-M relation with a total scatter that is lower by 40 per cent. From the distribution of the estimates of c200 and M200, with associated statistical (15-25%) and systematic (5-15%) errors, we use the predicted values from semi-analytic prescriptions calibrated through N-body numerical runs and measure sigma_8*Omega_m^(0.60+-0.03)= 0.45+-0.01 (at 2 sigma level, statistical only) for the subsample of the clusters where the mass reconstruction has been obtained more robustly, and sigma_8*Omega_m^(0.56+-0.04) = 0.39+-0.02 for the subsample of the 11 more relaxed LEC objects. With the further constraint from the fgas distribution in our sample, we break the degeneracy in the sigma_8-Omega_m plane and obtain the best-fit values sigma_8~1.0+-0.2 (0.75+-0.18 when the subsample of the more relaxed objects is considered) and Omega_m = 0.26+-0.01.
We test the predictions of Emergent Gravity using matter densities of relaxed, massive clusters of galaxies using observations from optical and X-ray wavebands. We improve upon previous work in this area by including the baryon mass contribution of the brightest cluster galaxy in each system, in addition to total mass profiles from gravitational lensing and mass profiles of the X-ray emitting gas from Chandra. We use this data in the context of Emergent Gravity to predict the apparent dark matter distribution from the observed baryon distribution, and vice-versa. We find that although the inclusion of the brightest cluster galaxy in the analysis improves the agreement with observations in the inner regions of the clusters ($r lesssim 10-30$ kpc), at larger radii ($r sim 100-200$ kpc) the Emergent Gravity predictions for mass profiles and baryon mass fractions are discrepant with observations by a factor of up to $sim2-6$, though the agreement improves at radii near $r_{500}$. At least in its current form, Emergent Gravity does not appear to reproduce the observed characteristics of relaxed galaxy clusters as well as cold dark matter models.
We propose a new cosmological framework in which the strength of the gravitational force acted on dark matter at late time can be weaker than that on the standard matter fields without introducing extra gravitational degrees of freedom. The framework integrates dark matter into a type-II minimally modified gravity that was recently proposed as a dark energy mimicker. The idea that makes such a framework possible consists of coupling a dark matter Lagrangian and a cosmological constant to the metric in a canonically transformed frame of general relativity (GR). On imposing a gauge fixing constraint, which explicitly breaks the temporal diffeomorphism invariance, we keep the number of gravitational degrees of freedom to be two, as in GR. We then make the inverse canonical transformation to bring the theory back to the original frame, where one can add the standard matter fields. This framework contains two free functions of time which specify the generating functional of the above mentioned canonical transformation and which are then used in order to realize desired time evolutions of both the Hubble expansion rate $H(z)$ and the effective gravitational constant for dark matter $G_{rm eff}(z)$. The aim of this paper is therefore to provide a new framework to address the two puzzles present in todays cosmology, i.e. the $H_0$ tension and the $S_8$ tension, simultaneously. When the dark matter is cold in this framework, we dub the corresponding cosmological model the V Canonical Cold Dark Matter (VCCDM), as the cosmological constant $Lambda$ in the standard $Lambda$CDM is replaced by a function $V(phi)$ of an auxiliary field $phi$ and the CDM is minimally coupled to the metric in a canonically transformed frame.
In this paper, we discuss a model-independent way to obtain the present dark matter density parameter ($Omega_{rm{c,0}}$) by combining gas mass fraction measurements in galaxy clusters ($f_{gas}$), type Ia supernovae (SNe Ia) observations and measurements of the cosmic baryon abundance from observations of absorption systems at high redshifts. Our estimate is $Omega_{rm{c,0}} = 0.244 pm 0.013$ ($1sigma$). By considering the latest local measurement of the Hubble constant, we obtain $Omega_{rm{M,0}} = 0.285 pm 0.013$ ($1sigma$) for the total matter density parameter. We also investigate departures of the evolution of the dark matter density with respect to the usual $a^{-3}$ scaling, as usual in interacting models of dark matter and dark energy. As the current data cannot confirm or rule out such an interaction, we perform a forecast analysis to estimate the necessary improvements in number and accuracy of upcoming $f_{gas}$ and SNe Ia observations to detect a possible non-minimal coupling in the cosmological dark sector.
We present results based on Chandra observations of a large sample of 129 hot galaxy clusters. We measure the concentration parameter c_200, the dark mass M_200 and the baryonic mass content in all the objects of our sample, providing the largest dataset of mass parameters for galaxy clusters in the redshift range z = 0.01 - 1.4. We confirm a that a tight correlation between c_200 and M_200, c propto M^a_vir /(1+z)^b with a = -0.56 +/- 0.15 and b =0.80 +/- 0.25 (68 per cent confidence limits), is present, in good agreement with the predictions from numerical simulations and previous observations. Fitting the mass profile with a generalized NFW model, we got the inner slope alpha, with alpha = 0.94 +/- 0.13. Finally, we show that the inner slope of the density profile, alpha correlates with the baryonic mass content, M_b : namely alpha is decreasing with increasing baryonic mass content.
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