DHOST gravity in Ultra-diffuse galaxies -- Part II: NGC 1052-DF4 and Dragonfly 44

Abstract

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.