High-Velocity Cloud Complex H: A Satellite of the Milky Way in a Retrograde Orbit?

Observations with the Green Bank Telescope of 21 cm H I emission from the high-velocity cloud complex H suggest that it is interacting with the Milky Way. A model in which the cloud is a satellite of the Galaxy in an inclined, retrograde circular orbit reproduces both the cloud's average velocity and its velocity gradient with latitude. The model places complex H at R=33+/-9 kpc from the Galactic center on a retrograde orbit inclined ~45° to the Galactic plane. At this location, it has a mass in H I of greater than 6×10⁶ M_solar and dimensions of at least 10×5 kpc. Some of the diffuse H I associated with the cloud has apparently been decelerated by interaction with Galactic gas. Complex H has similarities to the dwarf irregular galaxy Leo A and to some compact high-velocity clouds, and it has an internal structure nearly identical to parts of the Magellanic Stream, with a pressure P/k~100 cm^-3 K.