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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.
Gas response to the underlying stellar spirals is explored for M81 using unmagnetized hydrodynamic simulations. Constrained within the uncertainty of observations, 18 simulations are carried out to study the effects of selfgravity and to cover the pa
Recently, Buta etal. (2009) examined the question Do Bars Drive Spiral Density Waves?, an idea supported by theoretical studies and also from a preliminary observational analysis Block etal (2004). They estimated maximum bar strengths Q_b, maximum sp
Stationary density waves rotating at a constant pattern speed $Omega_{rm P}$ would produce age gradients across spiral arms. We test whether such age gradients are present in M81 by deriving the recent star formation histories (SFHs) of 20 regions ar
We present an analysis of the impact of spiral density waves (DWs) on the radial and surface density distributions of supernovae (SNe) in host galaxies with different arm classes. We use a well-defined sample of 269 relatively nearby, low-inclination
We investigate how the properties of spiral arms relate to other fundamental galaxy properties. To this end, we use previously published measurements of those properties, and our own measurements of arm-interarm luminosity contrasts for a large sampl