Longitude-velocity (l-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 axisymmetric potential. Several lines of evidence suggest that the Galaxy is barred, and gas flow in a barred potential could be consistent with the observed ``forbidden'' velocities and other features in the data. We compare the H I observations to l-V diagrams synthesized from 2-D fluid dynamical simulations of gas flows in a family of barred potentials. The gas flow pattern is very sensitive to the parameters of the assumed potential, which allows us to discriminate among models. We present a model that reproduces the outer contour of the H I l-V diagram reasonably well; this model has a strong 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/kpc, and matches the data best when viewed from 34\deg to the bar 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 galaxies, leads us to conclude that wide ranges of parameter space are incompatible with the observations. In particular we suggest that the bar must be fairly strong, must have an ILR, and cannot be too end-on, with the bar major axis at 35\deg +/- 5\deg to the line of sight. The H I data exhibit larger forbidden velocities over a wider longitude range than are seen in molecular gas; this important difference is the reason our favored model differs so significantly from other recently proposed models.
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机译:长期以来,人们一直知道内银河系中的H I和CO气体的经度-速度(I-V)图与轴对称电势中的圆周运动不一致。有几条证据表明,银河系被禁止了,处于禁止状态的气流可能与观测到的``禁止''速度和数据中的其他特征一致。我们将H I观测结果与由一系列禁止电势中的气流的二维流体动力学模拟合成的I-V图进行比较。气流模式对假定电势的参数非常敏感,这使我们能够区分模型。我们提出了一个模型,该模型可以很好地再现H I-V图的外部轮廓。该模型的半长轴为3.6 kpc,轴比约为3:1,内部Lindblad共振(ILR),图案速度为42 km / s / kpc,具有坚固的条形,并且与数据观察时最匹配从34度到钢筋长轴。这些模型的行为,再加上银河系中的冲击应类似于外部星系中的冲击的约束,使我们得出结论,认为参数空间的范围与观测值不兼容。特别是,我们建议该杆必须相当坚固,必须具有ILR且不能太末端,杆的主轴线应与视线成35 \ deg +/- 5 \ deg。 H I数据在较宽的经度范围内显示出比在分子气体中更大的禁止速度。这个重要的差异是我们偏爱的模型与其他最近提出的模型有如此大差异的原因。
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