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The formation and evolution of spiral arms in low surface brightness galaxies (LSBs) are not
well-understood. We study the dynamics of spiral arms in two prototypical LSBs, F568-VI and
F568-01, using both analytical models and N-body + hydrodynamical simulations. We first
consider the disk as a 2-component system of gravitationally-coupled stars and gas in the force
field of a \emph{spherical} dark matter halo, subjected to local, non-axisymmetric perturbations.
However, no local spirals are formed. We next assume the disk to be a 1-component system of
stars in the net gravitational potential of a galaxy with a spherical dark matter halo perturbed by
a global m=2 instability. In this case, the growth time for spiral formation was low, equal to 0.78
and 0.96 Gyrs respectively, corresponding to a few dynamical times of the galaxies. Finally, we
simulate the LSBs using the N-body + hydrodynamical simulation code RAMSES. Our results
show that a quadrupolar field associated with an oblate halo with an axial ratio of 0.7 is
necessary to drive a long-lived global spiral in the LSB disks. Further, feedback corresponding
to a supernova mass fraction of ~ 0.05 is essential to comply with the observed stellar surface
density. The simulated spirals survive for about ten dynamical times and the average pattern
speed lies between 10 - 15 kms -1 kpc -1 . The spiral arm thus formed is therefore a transient global
pattern driven by the tidal field of the oblate dark matter halo. |