Three-dimensional magnetohydrodynamic modeling of the gaseous structure of the galaxy:: Setup and initial results

We show the initial results of our three-dimensional MHD simulations of the flow of the Galactic atmosphere as it responds to a spiral perturbation in the potential. In our standard case, as the gas approaches the arm, there is a downward converging flow that terminates in a complex of shocks just ahead of the midplane density peak. The density maximum slants forward at high z, preceded by a similarly leaning shock. The latter diverts the flow upward and over the arm, as in a hydraulic jump. Behind the gaseous arm, the flow falls again, generating further secondary shocks as it approaches the lower z material. In cases with two arms in the perturbing potential, the gaseous arms tends to lie somewhat downstream of the potential minimum. In the four-arm case, this is true at large r or early evolution times. At smaller r, the gaseous arms follow a tighter spiral, crossing the potential maximum and fragmenting into sections arranged on average to follow the potential spiral. Structures similar to the high-z part of the gaseous arms are found in the interarm region of our two-arm case, while broken arms and low column density bridges are present in the four-arm case. Greater structure is expected when we include cooling of denser regions. We present three examples of what can be learned from these models. We compare the velocity field with that of purely circular rotation and find that an observer inside the Galaxy should see radial velocity deviations typically greater than 20 km s(-1). Synthetic spectra, vertical from the midplane, show features at velocities approximate to-20 km s(-1), which do not correspond to actual density concentrations. Placing the simulated observer outside the Galaxy, we find velocity structure and arm corrugation similar to those observed in Halpha in NGC 5427.