Astron. Astrophys. 344, 483-493 (1999)
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
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.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999