Abstract
Using N-body simulations, we study the properties of a bar induced in a dwarf galaxy as a result of tidal interaction with the Milky Way. The dwarf is initially composed of a disk embedded in a dark matter halo and we follow its evolution on a typical orbit for 10 Gyr. It undergoes an evolution typical of tidally stirred dwarf galaxies: it loses mass, the stellar component transforms from a disk to a spheroid and the rotation of the stars is partially replaced by random motions. A tidally induced bar forms at the first pericentre passage and survives until the end of the evolution. Fourier decomposition of the distribution of stars reveals that only even modes are significant and preserve a hierarchy so that the bar mode is always the strongest. They show a characteristic profile with a maximum, similar to simulated bars forming in isolated galaxies and observed bars in real galaxies. We adopt the maximum of the bar mode as a measure of the bar strength and we estimate the bar length by comparing the density profiles along the bar and perpendicular to it. Both the bar strength and the bar length decrease with time, mainly at pericentres, as a result of tidal torques acting at those times and not to secular evolution. The pattern speed of the bar varies significantly on a time scale of 1 Gyr and seems to be fully controlled by the orientation of the tidal torque from the Milky Way. The bar is never tidally locked but we discover a hint of a 5/2 orbital resonance between the third and fourth pericentre passage. The speed of the bar expressed in terms of the ratio of the bar length to the corotation radius decreases in the long run so that the bar changes from initially rather fast to slow in the later stages. The boxy/peanut shape is clearly visible in the edge-on view between the second and third pericentre and its occurrence is preceded by a short period of buckling instability.
Highlights
Observations indicate that a significant fraction of spiral galaxies are barred (e.g. Cheung et al 2013, and references therein)
We studied the formation and evolution of a stellar bar induced in a discy dwarf galaxy orbiting the Milky Way by tidal forces from the host
We measured the main properties of the bar such as its strength, length and pattern speed as a function of time and related the pattern speed to the dwarf’s circular frequency
Summary
Observations indicate that a significant fraction of spiral galaxies are barred (e.g. Cheung et al 2013, and references therein). While a simplified analytic description of the phenomenon is possible (Binney & Tremaine 2008), the problem can be fully tackled only using N-body simulations This approach was pioneered by Miller & Smith (1979) who studied the bar evolution and discussed the bar pattern speed, the particle orbits and the predicted observational properties of bars. A possible interpretation of these results is that dwarf galaxies are born with dynamically hotter discs which delays bar formation (Athanassoula & Sellwood 1986; Sheth et al 2012) until they are accreted by a more massive galaxy or become a member of a group or cluster where they are affected by tidal forces An example of such a tidally induced bar could be the one in M82, resulting from an interaction with M81 (Wills et al 2000).
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