Barred galaxies: Intrinsic or extrinsic?

A unified picture is presented of the formation of bar structures in disk galaxies of various morphological types. In order to discuss bar formation in the context of galactic disk formation, a simple analytic model is constructed of the growth of galactic disks by infall of primordial gas from halos and subsequent star formation in the disks. It is monitored during the course of disk growth whether or not the condition for spontaneous bar formation (i.e., bar instability) is fulfilled for the stellar disk component. It is found that the infall timescale is a key parameter that controls the dynamical property of the resulting stellar disk. Disks that grow fast by rapid infall experience gas-rich phases, in which massive gas clumps arising from gravitational instability in the gas disk heat the stellar disk component dynamically. When the disk has fully grown and becomes mostly stellar, it has already acquired enough random motions to suppress bar instability. On the other hand, when the gas infall from the halo region proceeds slowly, star formation (though less intense than in rapid infall cases) keeps gas mass in the disk low, leading to a dynamically cold stellar component due to lack of strong heating by massive gas clumps. Therefore, the stellar disk becomes unstable and forms a bar once its mass fraction relative to the total galaxy mass reaches a critical value. Based on this result, we propose that late-type barred galaxies, the disks of which are considered to have formed by slow accretion of the halo gas, have intrinsic origin, whereas the bars in early-type galaxies, whose disks are likely to have grown quickly, have been formed in tidal interactions with other galaxies. Numerical simulations have been carried out which show that the bars created by tidal perturbations tend to have a relatively flat density profile along the bar major axis with ''shoulders'' (abrupt steepening of the gradient) at the bar ends, whereas spontaneous bars have a steeper profile. The formation scenario described above, combined with this numerical result, can explain the observed dichotomy that early-type galaxies generally have a flat bar, while late-type galaxies have a bar of exponential type.