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Boxy/peanut (b/p) bulges, the vertically extended inner parts of bars, areubiquitous in barred galaxies in the local Universe, including our own MilkyWay. At the same time, a majority of external galaxies and the Milky Way alsopossess a thick-disc. However, the dynamical effect of thick-discs in the b/pformation and evolution is not fully understood. Here, we investigate theeffect of thick-discs in the formation and evolution of b/ps by using a suiteof N-body models of (kinematically cold) thin and (kinematically hot) thickdiscs. Within the suite of models, we systematically vary the mass fraction ofthe thick disc, and the thin-to-thick disc scale length ratio. The b/ps form inalmost all our models via a vertical buckling instability, even in the presenceof a massive thick disc. The thin disc b/p is much stronger than the thick discb/p. With increasing thick disc mass fraction, the final b/p structure getsprogressively weaker in strength and larger in extent. Furthermore, thetime-interval between the bar formation and the onset of buckling instabilitygets progressively shorter with increasing thick-disc mass fraction. Thebreaking and restoration of the vertical symmetry (during and after the b/pformation) show a spatial variation -- the inner bar region restores verticalsymmetry rather quickly (after the buckling) while in the outer bar region, thevertical asymmetry persists long after the buckling happens. Our findings alsopredict that at higher redshifts, when discs are thought to be thicker, b/pswould have more 'boxy-shaped' appearance than more 'X-shaped' appearance. Thisremains to be tested from future observations at higher redshifts.

Bars and boxy/peanut bulges in thin and thick discs III. Boxy/peanut bulge formation and evolution in presence of thick discs