The High-Velocity Clouds Above the Disk of the Outer Milky Way: Misty Precipitating Gas in a Region Roiled by Stellar Streams

Abstract

The high-velocity clouds (HVCs) in the outer Milky Way at 20 • < l < 190 • have similar spatial locations, metallicities, and kinematics. Moreover, their locations and kinematics are coincident with several extraplanar stellar streams. The HVC origins may be connected to the stellar streams, either stripped directly from them or precipitated by the aggregate dynamical roiling of the region by the stream progenitors. This paper suggests that these HVCs are "misty" precipitation in the stream wakes based on the following observations. New high-resolution (2.6 km s −1 ) ultraviolet spectroscopy of the QSO H1821+643 resolves what appears to be a single HVC absorption cloud (at 7 km s −1 resolution) into five components with T 3 × 10 4 K. Photoionization models can explain the low-ionization components but require some depletion of refractory elements by dust, and model degeneracies allow a large range of metallicity. High-ionization absorption lines (Si iv, C iv, and O vi) are kinematically aligned with the lowerionization lines and cannot be easily explained with photoionization or equilibrium collisional ionization; these lines are best matched by non-equilibrium rapidly cooling models, i.e., condensing/precipitating gas, with high metallicity and a significant amount of H i. Both the low-and high-ionization phases have low ratios of cooling time to freefall time and cooling time to sound-crossing time, which enables fragmentation and precipitation. The H1821+643 results are corroborated by spectroscopy of six other nearby targets that likewise show kinematically correlated low-and high-ionization absorption lines with evidence of dust depletion and rapid cooling.