Present-day Galactic Evolution: Low-metallicity, Warm, Ionized Gas
  Inflow Associated with High-Velocity Cloud Complex A
  The high-velocity cloud (HVC) Complex A is a probe of the physical conditions
in the Galactic halo. The kinematics, morphology, distance, and metallicity of
Complex A indicate that it represents new material that is accreting onto the
Galaxy. We present Wisconsin H-alpha Mapper (WHAM) kinematically resolved
observations of Complex A over the velocity range of -250 to -50 km/s in the
local standard of rest reference frame. These observations include the first
full H-alpha intensity map of Complex A across (l, b) = (124, 18) to (171, 53)
and deep targeted observations in H-alpha, [S II]6716, [N II]6584, and [O
I]6300 towards regions with high H I column densities, background quasars, and
stars. The H-alpha data imply that the masses of neutral and ionized material
in the cloud are similar, both being greater than a million solar masses. We
find that the Bland-Hawthorn & Maloney (1999, 2001) model for the intensity of
the ionizing radiation near the Milky Way is consistent with the known distance
of the high-latitude part of Complex A and an assumed cloud geometry that puts
the lower-latitude parts of the cloud at a distance of 7 to 8 kpc. This
compatibility implies a 5% ionizing photon escape fraction from the Galactic
disk. We also provide the nitrogen and sulfur upper abundance solutions for a
series of temperatures, metallicities, and cloud configurations for purely
photoionized gas; these solutions are consistent with the sub-solar abundances
found by previous studies, especially for temperatures above 10,000 K or for
gas with a high fraction of singly-ionized nitrogen and sulfur.
