Formation and fragmentation of gaseous spurs in spiral galaxies

Intermediate-scale spurs are common in spiral galaxies, but perhaps most distinctively evident in a recent image by Scoville & Rector showing a quasi-regular series of dust lanes projecting from the arms of M51. We investigate, using time-dependent numerical MHD simulations, how such spurs could form (and subsequently fragment) from the interaction of a gaseous interstellar medium with a stellar spiral arm. We model the gaseous medium as a self-gravitating, magnetized, differentially rotating, razor-thin disk. The basic flow shocks and compresses as it passes through a local segment of a tightly wound, trailing stellar spiral arm, modeled as a rigidly rotating gravitational potential. We first construct one-dimensional profiles for flows with spiral shocks. When the postshock Toomre parameter Q(sp) is sufficiently small, self-gravity is too large for one-dimensional steady solutions to exist. The critical values of Q(sp) are similar to0.8, 0.5, and 0.4 for our models with zero, subequipartition, and equipartition magnetic fields, respectively. We then study the growth of self-gravitating perturbations in fully two-dimensional flows and find that spur-like structures rapidly emerge in our magnetized models. We associate this gravitational instability with the magneto-Jeans mechanism, in which magnetic tension forces oppose the Coriolis forces that would otherwise prevent the coalescence of matter along spiral arms. The shearing and expanding velocity field shapes the condensed material into spurs as it flows downstream from the arms. Potentially, spur shapes and locations may be used to determine the spiral pattern speed. Although we find that swing amplification can help form spurs when the arm-interarm contrast is moderate, unmagnetized systems that are quasi-axisymmetrically stable are generally also stable to nonaxisymmetric perturbations, suggesting that magnetic effects are essential. In nonlinear stages of evolution, the spurs in our models undergo fragmentation to form similar to4x10(6) M-circle dot clumps, which we suggest could evolve into bright arm and interarm H II regions as seen in spiral galaxies.