Prof. Arunima Banerjee

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-1kpc-1. The spiral arm thus formed is therefore a transient global pattern driven by the tidal field of the oblate dark matter halo.