Spiral Density Waves in M81. I. Stellar Spiral Density Waves
  Aside from the grand-design stellar spirals appearing in the disk of M81, a
pair of stellar spiral arms situated well inside the bright bulge of M81 has
been recently discovered by Kendall et al. (2008). The seemingly unrelated
pairs of spirals pose a challenge to the theory of spiral density waves. To
address this problem, we have constructed a three component model for M81,
including the contributions from a stellar disk, a bulge, and a dark matter
halo subject to observational constraints. Given this basic state for M81, a
modal approach is applied to search for the discrete unstable spiral modes that
may provide an understanding for the existence of both spiral arms. It is found
that the apparently separated inner and outer spirals can be interpreted as a
single trailing spiral mode. In particular, these spirals share the same
pattern speed 25.5 km s$^{-1}$ kpc$^{-1}$ with a corotation radius of 9.03 kpc.
In addition to the good agreement between the calculated and the observed
spiral pattern, the variation of the spiral amplitude can also be naturally
reproduced.
