Suppression of gravitational instabilities by dominant dark matter halo
  in low surface brightness galaxies
  The low surface brightness galaxies are gas-rich and yet have a low star
formation rate, this is a well-known puzzle. The spiral features in these
galaxies are weak and difficult to trace, although this aspect has not been
studied much. These galaxies are known to be dominated by the dark matter halo
from the innermost regions. Here we do a stability analysis for the galactic
disc of UGC 7321, a low surface brightness, superthin galaxy, for which the
various observational input parameters are available. We show that the disc is
stable against local, linear axisymmetric and non-axisymmetric perturbations.
The Toomre Q parameter values are found to be large (>> 1) mainly due to the
low disc surface density and the high rotation velocity resulting due to the
dominant dark matter halo, which could explain the observed low star formation
rate. For the stars-alone case, the disc shows finite swing amplification but
the addition of dark matter halo suppresses that amplification almost
completely. Even the inclusion of the low-dispersion gas which constitutes a
high disc mass fraction does not help in causing swing amplification. This can
explain why these galaxies do not show strong spiral features. Thus the
dynamical effect of a halo that is dominant from inner regions can naturally
explain why star formation and spiral features are largely suppressed in low
surface brightness galaxies, making these different from the high surface
brightness galaxies.
