Disk stability under MONDian gravity

Abstract

Toomre's $Q$ stability parameter has long been shown through various theoretical arguments and numerical simulations, to be the principal determinant of stability against self-gravity in a galactic disk, under classical gravity. Comparison with observations however, has not always confirmed the condition of $Q=Q_{crit}$ to be well correlated with various critical galactic radii. In this paper we derive the analogous critical parameter, $Q_{M}$, under MONDian gravity. The result is a modification by a factor of $(\sigma \Omega)/a_{0}$, $Q_{M}=(\sigma \Omega)^{2}/(a_{0}G \Sigma)$, where $a_{0}$ is the critical acceleration scale of MOND. We then show through a direct comparison to a homogeneous sample of observed disk galaxies with measured brightness profiles, rotation velocity curves and internal velocity dispersion profiles, that the critical radii at which brightness profiles dip below the exponential fit, are much more accurately predicted by $Q_{M}$ than by the $Q$ of classical gravity. This provides a new and completely independent argument supporting the reality of a change in the form of gravity on reaching the low acceleration regime.