How does a low surface brightness galaxy form spiral arms?

TL;DR

This study investigates the formation of spiral arms in low surface brightness galaxies through analytical models and simulations, revealing that oblate dark matter halos and supernova feedback are key to long-lived spiral structures.

Contribution

It demonstrates that a quadrupolar halo field and supernova feedback are essential for forming and sustaining spiral arms in LSBs, providing new insights into their dynamics.

Findings

Oblate halo with axial ratio 0.7 drives spiral arms.

Supernova feedback with 0.05 mass fraction is necessary.

Spiral arms last about ten dynamical times, with pattern speeds 10-15 km/s/kpc.

Abstract

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 \emph{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. \emph{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 $\sim$ 0.05 is essential to comply with the observed stellar surface density. The simulated spirals survives for about ten dynamical times and the average pattern speed lies between 10 - 15 $\rm{kms^{-1}{kpc}^{-1}}$. The spiral arm thus formed is therefore a transient global pattern driven by the tidal field of the oblate dark matter halo.