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Register a new userNo need for dark matter in galaxy clusters within Galileon theory
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Vincenzo Salzano Dr.
Publication date
2016
fields
Physics
and research's language is
English
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Modified gravity theories with a screening mechanism have acquired much interest recently in the quest for a viable alternative to General Relativity on cosmological scales, given their intrinsic property of being able to pass Solar System scale tests and, at the same time, to possibly drive universe acceleration on much larger scales. Here, we explore the possibility that the same screening mechanism, or its breaking at a certain astrophysical scale, might be responsible of those gravitational effects which, in the context of general relativity, are generally attributed to Dark Matter. We consider a recently proposed extension of covariant Galileon models in the so-called beyond Horndeski scenario, where a breaking of the Vainshtein mechanism is possible and, thus, some peculiar observational signatures should be detectable and make it distinguishable from general relativity. We apply this model to a sample of clusters of galaxies observed under the textit{CLASH} survey, using both new data from gravitational lensing events and archival data from X-ray intra-cluster hot gas observations. In particular, we use the latter to model the gas density, and then use it as the only ingredient in the matter clusters budget to calculate the expected lensing convergence map. Results show that, in the context of this extended Galileon, the assumption of having only gas and no Dark Matter at all in the clusters is able to match observations. We also obtain narrow and very interesting bounds on the parameters which characterize this model. In particular, we find that, at least for one of them, the general relativity limit is excluded at $2sigma$ confidence level, thus making this model clearly statistically different and competitive with respect to general relativity.
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In recent years, significant progress has been made in building new galaxy clusters samples, at low and high redshifts, from wide-area surveys, particularly exploiting the Sunyaev--Zeldovich (SZ) effect. A large effort is underway to identify and characterize these new systems with optical/NIR and X-ray facilities, thus opening new avenues to constraint cosmological models using structure growth and geometrical tests. A census of galaxy clusters sets constraints on reionization mechanisms and epochs, which need to be reconciled with recent limits on the reionization optical depth from cosmic microwave background (CMB) experiments. Future advances in SZ effect measurements will include the possibility to (unambiguously) measure directly the kinematic SZ effect, to build an even larger catalogue of galaxy clusters able to study the high redshift universe, and to make (spatially-)resolved galaxy cluster maps with even spectral capability to (spectrally-)resolve the relativistic corrections of the SZ effect.
Dark matter-dominated cluster-scale halos act as an important cosmological probe and provide a key testing ground for structure formation theory. Focusing on their mass profiles, we have carried out (gravity-only) simulations of the concordance LCDM cosmology, covering a mass range of 2.10^{12}-2.10^{15} solar mass/h and a redshift range of z=0-2, while satisfying the associated requirements of resolution and statistical control. When fitting to the Navarro-Frenk-White profile, our concentration-mass (c-M) relation differs in normalization and shape in comparison to previous studies that have limited statistics in the upper end of the mass range. We show that the flattening of the c-M relation with redshift is naturally expressed if c is viewed as a function of the peak height parameter, u. Unlike the c-M relation, the slope of the c- u relation is effectively constant over the redshift range z=0-2, while the amplitude varies by ~30% for massive clusters. This relation is, however, not universal: Using a simulation suite covering the allowed wCDM parameter space, we show that the c- u relation varies by about +/- 20% as cosmological parameters are varied. At fixed mass, the c(M) distribution is well-fit by a Gaussian with sigma_c/c = 0.33, independent of the radius at which the concentration is defined, the halo dynamical state, and the underlying cosmology. We compare the LCDM predictions with observations of halo concentrations from strong lensing, weak lensing, galaxy kinematics, and X-ray data, finding good agreement for massive clusters (M > 4.10^{14} solar mass/h), but with some disagreements at lower masses. Because of uncertainty in observational systematics and modeling of baryonic physics, the significance of these discrepancies remains unclear.
We derive a model for Sunyaev--Zeldovich data from a galaxy cluster which uses an Einasto profile to model the clusters dark matter component. This model is similar to the physical models for clusters previously used by the Arcminute Microkelvin Imager (AMI) consortium, which model the dark matter using a Navarro-Frenk-White (NFW) profile, but the Einasto profile provides an extra degree of freedom. We thus present a comparison between two physical models which differ only in the way they model dark matter: one which uses an NFW profile (PM I) and one that uses an Einasto profile (PM II). We illustrate the differences between the models by plotting physical properties of clusters as a function of cluster radius. We generate AMI simulations of clusters which are textit{created} and textit{analysed} with both models. From this we find that for 14 of the 16 simulations, the Bayesian evidence gives no preference to either of the models according to the Jeffreys scale, and for the other two simulations, weak preference in favour of the correct model. However, for the mass estimates obtained from the analyses, the values were within $1sigma$ of the input values for 14 out of 16 of the clusters when using the correct model, but only in 6 out of 16 cases when the incorrect model was used to analyse the data. Finally we apply the models to real data from cluster A611 obtained with AMI, and find the mass estimates to be consistent with one another except in the case of when PM II is applied using an extreme value for the Einasto shape parameter.
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 $R_{500}$.
The study of the cross-correlation angular power spectrum between gravitational tracers and electromagnetic signals can be a powerful tool to constrain Dark Matter (DM) microscopic properties. In this work we correlate Fermi diffuse g-ray maps with catalogues of galaxy clusters. To emphasize the sensitivity to a DM signal, we select clusters at low-redshift $0<z<0.2$ and with large-halo mass $M_{500}>10^{13}M_odot$. The analysis is performed with four catalogues in different wavebands, including infrared, optical and X-rays. No evidence for a DM signal is identified. On the other hand, we derive competitive bounds: the thermal cross-section is excluded at 95% C.L. for DM masses below 20 GeV and annihilation in the $tau^+-tau^-$ channel.
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