Dwarf irregular galaxies with extended HI gas disks: Suppression of
  small-scale spiral structure by dark matter halo
  Dwarf irregular galaxies with extended HI disk distributions, such as DDO
154, allow measurement of rotation curves, hence deduction of dark matter halo
properties to large radial distances, up to several times the optical radius.
These galaxies contain a huge reservoir of dark matter halo, which dominates
over most of disk. We study the effect of the dark matter halo on small-scale
spiral features by carrying out the local, non-axisymmetric perturbation
analysis in the disks of five such late-type, gas-rich dwarf irregular
galaxies, namely, DDO 154, NGC 3741, DDO 43, NGC 2366, and DDO 168 which host a
dense and compact dark matter halo. We show that when the gas disk is treated
alone, it allows a finite swing amplification; which would result in
small-scale spiral structure in the outer gas disk, but the addition of dark
matter halo in the analysis results in a higher Toomre Q parameter which
prevents the amplification almost completely. This trend is also seen to be
true in regions inside the optical radius. This implies absence of strong
small-scale spiral arms in these galaxies, which is in agreement with
observations. Hence despite being gas-rich, and in fact having gas as the main
baryonic component, these galaxies cannot support small-scale spiral structure
which would otherwise have been expected in normal gas-rich galaxies.
