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 .
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 .