Bars and boxy/peanut bulges in thin and thick discs

Abstract

Boxy/peanut (b/p) bulges, the vertically extended inner parts of bars, are ubiquitous in barred galaxies in the local Universe, including our own Milky Way. At the same time, the majority of external galaxies and the Milky Way also possess a thick disc. However, the dynamical effect of thick discs in the b/p formation and evolution is not fully understood. Here, we investigate the effect of thick discs in the formation and evolution of b/ps by using a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs. Within the suite of models, we systematically vary the mass fraction of the thick disc, and the thin-to-thick disc scale length ratio. The b/ps form in almost all our models via a vertical buckling instability, even in the presence of a massive thick disc. The thin disc b/p is much stronger than the thick disc b/p. With an increasing thick-disc mass fraction, the final b/p structure becomes progressively weaker in strength and larger in extent. Furthermore, the time interval between the bar formation and the onset of buckling instability becomes progressively shorter with an increasing thick-disc mass fraction. The breaking and restoration of the vertical symmetry (during and after the b/p formation) show a spatial variation – the inner bar region restores vertical symmetry rather quickly (after the buckling), while in the outer bar region the vertical asymmetry persists long after the buckling happens. Our findings also predict that at higher redshifts, when discs are thought to be thicker, b/ps would have a more “boxy” appearance than an “X-shaped” one. This remains to be tested in future observations at higher redshifts.