A Component of the Smith High Velocity Cloud Now Crossing the Galactic Plane

Abstract

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 degrees. Using new 21cm HI 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 VLSR 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 HI mass may be as high as 10^6 Solar masses, 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 ISM 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 degrees (about 7 kpc) across the sky, in two pieces separated by a gap of about 1 kpc.

Accepted for publication in the Astrophysical Journal

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