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Astron. Astrophys. 344, 483-493 (1999)

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4. Conclusions

The main conclusions drawn from our study of bar-induced mass relocation in galactic discs are:

  • Bar formation will radically alter the surface density profile of a disc. The new profile will have three domains: a region with a steep gradient within the bar area, a region with a very shallow gradient outside the bar extending out to 2-2.5 bar lengths, and a region with a normal gradient further out. The first two domains have exponential profiles while the last one is Kuzmin-like.

  • Matter in the inner parts of a galaxy migrates inwards, while matter in the outer parts migrate outwards. The division line appears close to the turn-over radius.

  • The largest depopulation occurs in an area just outside the bar region, where the number of stars may go down by two-thirds in a few Gyrs. The largest increase in surface density occurs in the outskirts of the galactic disk.

  • Stars from an area just outside the bar region will be spread out all over the galaxy. The outer parts of a galaxy will thus consist of two dynamical populations of stars: those originally born there orbiting on roughly circular orbits, and stars ejected from the area outside the bar region orbiting on highly eccentric orbits.

  • Orbits with pericentre distances between 1 and 2 bar lengths are subjected to strong perturbations from the bar at each pericentre passage. The shape of orbits can, as a result of this, change dramatically over just a few revolutions.

  • The location of the pericentre passage with respect to the bar will determine whether the passage will be an accelerating one, boosting the star into a more elongated orbit, or a decelerating one, reducing the apocentre distance.

  • Stars can even be ejected to distances far outside the disc. The mechanism behind these ejections is a series of "fortunate" pericentre passages, each one boosting the star into a more elongated orbit. The process is fast; stars can reach out to distances corresponding to 10 bar lengths in less than 2 Gyr from the time of bar formation.

  • Ejection of stars from barred galaxies may thus contribute to the population of intergalactic stars recently found. Stars could either be accelerated to escape velocity by the bar itself or sent out to such large distances that neighbouring galaxies easily could perturb them into escape trajectories.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999