Bar instability in cosmological halos

Aims. We investigate the growth of bar instability in stellar disks embedded in a dark matter halo evolving in a fully consistent cosmological framework. Methods. We perform seven cosmological simulations to emphasise the role of both the disk-to-halo mass ratio and of the Toomre parameter, Q, on the evolution of the disk. We also compare Our fully cosmological cases with corresponding isolated simulations where the same halo is extracted from the cosmological scenario and evolved in physical coordinates. Results. A long living bar, lasting about 10 Gyr, appears in all Our simulations. In particular, disks expected to be stable according to classical criteria form weak bars. We argue that such a result is due to the dynamical properties of our cosmological halo which is far from stability and isotropy, typical of the classical halos used in the literature; it is dynamically active, has substructures and undergoes infall. Conclusions. At least for mild self-gravitating disks, the study of the bar instability using isolated isotropic halos in gravitational equilibrium can lead to misleading results. Furthermore, the cosmological framework is needed to quantitatively investigate such an instability.