Central-surface-densities correlation in general MOND theories

Abstract

It is shown that the foundational axioms of MOND alone predict a strong correlation between a bulk measure of the baryonic surface density, $\Sigma_B$, and the corresponding dynamical one, $\Sigma_D$, of an isolated object, such as a galaxy. The correlation is encapsulated by its high- and low-$\Sigma_B$ behaviors. For $\Sigma_B\gg\Sigma_M\equiv a_0/2\pi G$ ($\Sigma_M$ is the critical MOND surface density) one has $\Sigma_D\approx\Sigma_B$. Their difference -- which would be interpreted as the contribution of dark matter -- is $\Sigma_P=\Sigma_D-\Sigma_B\sim\Sigma_M\ll\Sigma_B$. In the deep-MOND limit, $\Sigma_B\ll\Sigma_M$, one has $\Sigma_D\sim (\Sigma_M\Sigma_B)^{1/2}$. This is a primary prediction of MOND, shared by all theories that embody its basic tenets. Sharper correlations, even strict algebraic relations, $\Sigma_D(\Sigma_B)$, are predicted in specific MOND theories, for specific classes of mass distribution -- e.g., pure discs, or spherical systems -- and for specific definitions of the surface densities. I proceed to discuss such tighter correlations for the central surface densities of axisymmetric galactic systems, $\Sigma^0_B$ and $\Sigma^0_D$. Past work has demonstrated such relations for pure discs in the AQUAL and QUMOND theories. Here I consider them in broader classes of MOND theories. For most observed systems, $\Sigma^0_D$ can not be determined directly at present, but, in many cases, a good proxy for it is the acceleration integral $\mathcal{G}\equiv\int_0^\infty g_r d\ln~r$, where $g_r$ is the radial acceleration along a reflection-symmetry plane of a system, such as a disc galaxy. $\mathcal{G}$ can be determined directly from the rotation curve. I discuss the extent to which $\mathcal{G}$ is a good proxy for $\Sigma^0_D$, and how the relation between them depends on system geometry, from pure discs, through disc-plus-bulge ones, to quasi-spherical systems.