Global Modeling of Spur Formation in Spiral Galaxies

We investigate the formation of substructure in spiral galaxies using global MHD simulations, including gas self-gravity. Local modeling by Kim and Ostriker (2002) previously showed that self-gravity and magnetic fields cause rapid growth of overdensities in spiral arms; differential compression of gas flowing through the arms then results in formation of sheared structures in the interarms. These sheared structures resemble features described as spurs or feathers in optical and IR observations of many spiral galaxies. Global modeling extends previous local models by including the full effects of curvilinear coordinates, a realistic log-spiral perturbation, self-gravitational contribution from 5 radial wavelengths of the spiral shock, and variation of density and epicyclic frequency with radius. We show that with realistic Toomre Q values, self-gravity and galactic differential rotation produce filamentary gaseous structures with kpc-scale separations, regardless of the strength – or even presence – of a stellar spiral potential. However, a sufficiently strong spiral potential is required to produce “true spurs”, consisting of interarm structures emerging from gas concentrations in the main spiral arms. In models where Q is initially constant, filaments due to interarm self-gravity grow mainly in the outer regions, whereas true arm spurs grow only in the inner regions. For models with Q ∝ R, outer regions are intrinsically more stable so “background” interarm filaments do not grow, but arm spurs can develop if the spiral potential is strong. Unlike independently-growing “background” filaments, the orientation of arm spurs depends on galactic location. Inside corotation, spurs emanate outward, on the convex side of the arm; outside corotation, spurs grow inward, on the concave side of the arm. Based on orientation and the relation to arm clumps, it is possible to distinguish “true spurs” that originate as instabilities in the arms from independently growing “background” filaments. We measure spur spacings of ∼3 5 times the Jeans length in the arm,