A dynamical mechanism for the origin of nuclear rings

Abstract

We develop a dynamical theory for the origin of nuclear rings in barred\ngalaxies. In analogy with the standard theory of accretion discs, our theory is\nbased on shear viscous forces among nested annuli of gas. However, the fact\nthat gas follows non circular orbits in an external barred potential has\nprofound consequences: it creates a region of reverse shear in which it is\nenergetically favourable to form a stable ring which does not spread despite\ndissipation. Our theory allows us to approximately predict the size of the ring\ngiven the underlying gravitational potential. The size of the ring is loosely\nrelated to the location of the Inner Lindblad Resonance in the epicyclic\napproximation, but the predicted location is more accurate and is also valid\nfor strongly barred potentials. By comparing analytical predictions with the\nresults of hydrodynamical simulations, we find that our theory provides a\nviable mechanism for ring formation if the effective sound speed of the gas is\nlow ($\\cs\\lesssim1\\kms$), but that nuclear spirals/shocks created by pressure\ndestroy the ring when the sound speed is high ($\\cs\\simeq10\\kms$). We conclude\nthat whether this mechanism for ring formation is relevant for real galaxies\nultimately depends on the effective equation of state of the ISM. Promising\nconfirmation comes from simulations in which the ISM is modelled using\nstate-of-the-art cooling functions coupled to live chemical networks, but more\ntests are needed regarding the role of turbulence driven by stellar feedback.\nIf the mechanism is relevant in real galaxies, it could provide a powerful tool\nto constrain the gravitational potential, in particular the bar pattern speed.\n