The role of density breaks in driving spiral structure in disc galaxies
arxiv(2024)
摘要
It is well established that stellar discs are destabilized by sharp features
in their phase space, driving recurrent spiral modes. We explore the extent to
which surface-density breaks in disc galaxies - which represent sharp changes
in the gradient of the disc density - drive new spiral modes. We employ linear
perturbation theory to investigate how disc breaks alter the eigenmode spectrum
of an otherwise pure exponential disc. We find that the presence of a density
break gives rise to a set of new, vigorously growing, modes. For a given
multiplicity, these edge modes occur in pairs, with closely separated
resonances between each pair. The growth rate of edge modes decreases when the
break is weakened or moved outward to lower-density regions of the disc. Both
down- and up-bending profiles excite edge modes, whose origin can be best
understood via the gravitational torques they exert on the underlying disc.
When the profile is down-bending (Type II) the faster growing mode is the inner
one while in the up-bending (Type III) case the outer mode is faster growing.
In both cases the faster growing mode has a corotation almost coincident with
the break. We show that the torques of the edge modes tend to smoothen the
break.
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