Insight into the baryon-gravity relation in galaxies

Abstract

Observations of spiral galaxies strongly support a one-to-one analytical relation between the inferred gravity of dark matter at any radius and the enclosed baryonic mass. It is baffling that baryons manage to settle the dark matter gravitational potential in such a precise way, leaving no ``messy'' fingerprints of the merging events and ``gastrophysical'' feedbacks expected in the history of a galaxy in a concordance Universe. This correlation of gravity with baryonic mass can be interpreted from several nonstandard angles, especially as a modification of gravity called $\mathrm{T}e\mathrm{V}e\mathrm{S}$, in which no galactic dark matter is needed. In this theory, the baryon-gravity relation is captured by the dieletric-like function $\ensuremath{\mu}$ of modified Newtonian dynamics (MOND), controlling the transition from $1/{r}^{2}$ attraction in the strong gravity regime to $1/r$ attraction in the weak regime. Here, we study this $\ensuremath{\mu}$-function in detail. We investigate the observational constraints upon it from fitting galaxy rotation curves, unveiling the degeneracy between the stellar mass-to-light ratio and the $\ensuremath{\mu}$-function as well as the importance of the sharpness of transition from the strong to weak gravity regimes. We also numerically address the effects of nonspherical baryon geometry in the framework of nonlinear $\mathrm{T}e\mathrm{V}e\mathrm{S}$, and exhaustively examine how the $\ensuremath{\mu}$-function connects with the free function of that theory. In that regard, we exhibit the subtle effects and wide implications of renormalizing the gravitational constant. We finally present a discontinuity-free transition between quasistatic galaxies and the evolving Universe for the free function of $\mathrm{T}e\mathrm{V}e\mathrm{S}$, inevitably leading to a return to $1/{r}^{2}$ attraction at very low accelerations in isolated galaxies.