The High-Velocity Clouds Above the Disk of the Outer Milky Way: Misty
  Precipitating Gas in a Region Roiled by Stellar Streams
  The high-velocity clouds (HVCs) in the outer Milky Way at $20^{\circ} < l <
190^{\circ}$ 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) ultraviolet spectroscopy
of the QSO H1821+643 resolves what appears to be a single HVC absorption cloud
(at 7 km/s resolution) into five components with $T \lesssim 3\times 10^{4}$ 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 (SiIV, CIV, and
OVI) are kinematically aligned with the lower-ionization 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
HI. 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.
