Extended Theories of Gravity with additional scalar degrees of freedom have recently acquired increasing interest due to the presence of a screening mechanism that allows suppressing at small scales (e.g., the Solar System scale) every modification restoring General Relativity. In this work, we consider a second-order Extended Theory of Gravity belonging to the family of Degenerate High Order Scalar Tensor theories (DHOST) characterized by a partial breaking of the Vainshtein screening mechanism. We study this model in two different scenarios as a description of dark energy only and as a description of both dark matter and dark energy. Such scenarios have been tested here by analysing a sample of 16 high-mass galaxy clusters targeted by the Cluster Lensing and Supernova survey with Hubble (CLASH) program using two complementary probes, namely X-ray and strong-and-weak gravitational lensing observations. In mass modelling, we adopt a multi-component approach including hot gas and galactic stellar contributions. For the majority of the clusters in our sample, results show mild Bayesian evidence in favour of the DHOST model as a description of dark energy over General Relativity. This model also appears to alleviate the discrepancy present in General Relativity between X-ray hydrostatic and lensing mass estimates. For the second scenario where gravity acts as both dark energy and dark matter due to the partial breaking of the Vainshtein screening mechanism at cluster scales, the model is statistically disfavoured compared to General Relativity.
The Ultra-Diffuse galaxy NGC1052-DF2 has recently been revealed to be “extremely deficient” in dark matter, if not lacking it at all. This claim has raised many questions regarding the relationship between baryons and dark matter in Ultra-Diffuse galaxies. But there seems to be a quite unanimous belief that, if such very low dark matter content is confirmed and extended to other similar galactic objects, it might be a deathblow to theories which modify and extend General Relativity. Deficient dark matter galaxies thus represent a fertile ground to test both standard dark matter and modified gravity theories. In this work, we consider a specific Degenerate Higher-Order Scalar Tensor model to study the internal kinematics of NGC1052-DF2. Due to the partial breaking of the corresponding screening mechanism, this model can possibly have large cosmological scale effects influencing the dynamics of smaller structures like galaxies. We consider two scenarios: one in which the model only describes dark energy; and one in which it additionally entirely substitutes dark matter. We find that the best model to explain data is General Relativity with only stellar contribution. But while in the former scenario General Relativity is still statistically favoured, in the latter one the alternative model is as much successful and effective as General Relativity in matching observations. Thus, we can conclude that even objects like NGC1052-DF2 are not in contrast, and are not obstacles, to the study and the definition of a reliable alternative to General Relativity.
Ultra-Diffuse galaxies are a family of gravitational systems with quite varied properties. On one hand we have cases like NGC1052-DF2 and NGC1052-DF4, both observed by the Dragonfly Array Telescope, which are claimed to be highly-deficient in dark matter. On the other hand, we have also observed Ultra-Diffuse galaxies which are almost totally dominated by dark matter, such as DF44, which is estimated to be at 99% dark. To explain such kind of a variety of behaviors might be a problem for both the standard dark matter paradigm and for alternative theories of gravity that try to overcome dark matter existence by modifying General Relativity.Here we consider a modified gravity theory belonging to the family of Degenerate Higher-Order Scalar Tensor theories to study the internal kinematics of both NGC1052-DF4 and DF44. The peculiarity of the chosen model is the partial breaking of its Vaishtein screening mechanism for which it might have an influence not only on cosmological scales but also on astrophysical ones. We consider two different possibilities: one in which the model only plays the role of dark energy; and another one in which it might also mimic a sort of effective dark matter.We get conflicting results. For NGC1052-DF4 we confirm that the galaxy dynamics might be successfully described even only by a stellar component and that, at least at the scale which are probed, the content of dark matter is quite low. In addition to that, we also show that the alternative gravity model is totally consistent with data and is statistically equivalent to a standard General Relativity dark matter scenario, and it might even replace dark matter. On the contrary, DF44 requires dark matter both in General Relativity and in our alternative gravity model. When the latter is considered only as a cosmological dark energy fluid, it is statistically fully reliable and equivalent to General Relativity. But when we try to use it to substitute dark matter, although we get good fits to the data, the constraints on the theoretical parameters are in sharp contrast with those derived from more stringent probes from the stellar scales.
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