The properties of the galactic bar implied by gas kinematics in the inner Milky Way

Longitude-velocity (E-V) diagrams of H I and CO gas in the inner Milky Way have long been known to be inconsistent with circular motion in an axisymme tric potential. Several lines of evidence suggest that the Galaxy is barred , and gas how in a barred potential could be consistent with the observed " forbidden" velocities and other features in the data. We compare the H I ob servations to l-V diagrams synthesized from two-dimensional fluid dynamical simulations of gas flows in a family of barred potentials. The gas flow pa ttern is very sensitive to the parameters of the assumed potential, which a llows us to discriminate among models. We present a model that reproduces t he outer contour of the H I l-V diagram reasonably well; this model has a s trong bar with a semimajor axis of 3.6 kpc, an axis ratio of approximately 3:1, an inner Lindblad resonance (ILR), and a pattern speed of 42 km s(-1) kpc(-1), and it matches the data best when viewed from 34 degrees to the ba r major axis. The behavior of the models, combined with the constraint that the shocks in the Milky Way bar should resemble those in external barred g alaxies, leads us to conclude that wide ranges of parameter space are incom patible with the observations. In particular we suggest that the bar must b e fairly strong, must have an ILR, and cannot be too end-on, with the bar m ajor axis at 35 degrees +/- 5 degrees to the line of sight. The H I data ex hibit larger forbidden velocities over a wider longitude range than are see n in molecular gas; this important difference is the reason our favored mod el differs so significantly from other recently proposed models.