A Component of the Smith High-velocity Cloud Now Crossing the Galactic Plane

We have identified a new structure in the Milky Way: a leading component of the Smith high-velocity cloud that is now crossing the Galactic plane near longitude 25°. Using new 21 cm H I data from the Green Bank Telescope (GBT), we measured the properties of several dozen clouds that are part of this structure. Their kinematics is consistent with that of the Smith Cloud with a V _LSR exceeding that permitted by circular rotation in their direction. Most of the clouds in the leading component show evidence that they are interacting with disk gas allowing the location of the interaction to be estimated. The leading component crosses the Galactic plane at a distance from the Sun of 9.5 kpc, about 4.5 kpc from the Galactic Center. Its H I mass may be as high as 10⁶ M _⊙, comparable to the mass of the neutral component of the Smith Cloud, but only a fraction of this is contained in clouds that are resolved in the GBT data. Like the Smith Cloud, the leading component appears to be adding mass and angular momentum to the interstellar medium in the inner Galaxy. We suggest that the Smith Cloud is not an isolated object, but rather part of a structure that stretches more than 40° (~7 kpc) across the sky, in two pieces separated by a gap of ~1 kpc.