Bars and boxy/peanut bulges in thin and thick discs

Abstract

The Milky Way and a majority of external galaxies possess a thick disc. However, the dynamical role of the (geometrically) thick disc in the bar formation and evolution is not fully understood. Here, we investigate the effect of thick discs in the formation and evolution of bars by means of a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs. We systematically varied the mass fraction of the thick disc, the thin-to-thick disc scale length ratio, and the thick disc scale height to examine the bar formation under diverse dynamical scenarios. Bars form almost always in our models, even in the presence of a massive thick disc. The part of the bar that consists of the thick disc closely follows the overall growth and temporal evolution of the part of the bar that consists of the thin disc, but the part of the bar in the thick disc is weaker than the part of the bar in the thin disc. The formation of stronger bars is associated with a simultaneous greater loss of angular momentum and a more intense radial heating. In addition, we demonstrate a preferential loss of angular momentum and a preferential radial heating of disc stars in the azimuthal direction within the extent of the bar in both thin and thick disc stars. For purely thick-disc models (without any thin disc), the bar formation critically depends on the disc scale length and scale height. A larger scale length and/or a larger vertical scale height delays the bar formation time and/or suppresses the bar formation almost completely in thick-disc-only models. We find that the Ostriker-Peeble criterion predicts the bar instability scenarios in our models better than the Efstathiou-Lake-Negroponte criterion.