Gas Dynamics in the Milky Way: Second Pattern Speed and Large–Scale Morphology

We present new gas flow models for the Milky Way inside the solar circle. We use SPH simulations in gravitational potentials determined from the NIR luminosity distribution of the bulge and disk, assuming constant NIR mass-to-light ratio, with an outer halo added in some cases. The luminosity models are based on the COBE/DIRBE maps and on clump giant star counts in several bulge fields, and include a spiral arm model for the disk. Gas flows in models which include massive spiral arms clearly match the observed CO (l, v) diagram better than if the potential does not include spiral structure. Furthermore, models in which the luminous mass distribution and the gravitational potential of the Milky Way have four spiral arms are better fits to the observed (l, v) diagram than two-armed models. Besides single pattern speed models we investigate models with separate pattern speeds for the bar and spiral arms. The most important difference is that in the latter case the gas spiral arms go through the bar corotation region, keeping the gas aligned with the arms there. In the (l, v) plot this results in characteristic regions which appear to be nearly void of gas. In single pattern speed models these regions are filled with gas because the spiral arms dissolve in the bar corotation region. Comparing with the CO data we find evidence for separate pattern speeds in the Milky Way. From a series of models the preferred range for the bar pattern speed is Ωp = 60± 5Gyr , corresponding to corotation at 3.4± 0.3 kpc. The spiral pattern speed is less well constrained, but our preferred value is Ωsp ≈ 20Gyr . A further series of gas models is computed for different bar angles, using separately determined luminosity models and gravitational potentials in each case. We find acceptable gas models for 20◦ ∼ < φbar ∼ < 25◦. The model with (φbar = 20 ◦, Ωp = 60Gyr , Ωsp = 20Gyr) gives an excellent fit to the spiral arm ridges in the observed (l, v) plot.