Effect of dark matter halo on global spiral modes in a collisionless
  galactic disc
  Low surface brightness (LSB) galaxies are dominated by dark matter halo from
the innermost radii; hence they are ideal candidates to investigate the
influence of dark matter on different dynamical aspects of spiral galaxies.
Here, we study the effect of dark matter halo on grand-design, m = 2, spiral
modes in a galactic disk, treated as a collisionless system, by carrying out a
global modal analysis within the WKB approximation. First, we study a
superthin, LSB galaxy UGC 7321 and show that it does not support discrete
global spiral modes when modeled as a disk-alone system or as a disk plus dark
matter system. Even a moderate increase in the stellar central surface density
does not yield any global spiral modes. This naturally explains the observed
lack of strong large-scale spiral structure in LSBs. An earlier work (Ghosh,
Saini & Jog, 2016) where the galactic disk was treated as a fluid system for
simplicity had shown that the dominant halo could not arrest global modes. We
found that this difference arises due to the different dispersion relation used
in the two cases and which plays a crucial role in the search for global spiral
modes. Thus the correct treatment of stars as a collisionless system as done
here results in the suppression of global spiral modes, in agreement with the
observations. We performed a similar modal analysis for the Galaxy, and found
that the dark matter halo has a negligible effect on large-scale spiral
structure.
