Exploring Hydrodynamic Instabilities along the Infalling High-Velocity
  Cloud Complex A
  Complex A is a high-velocity cloud that is traversing through the Galactic
halo toward the Milky Way's disk. We combine both new and archival Green Bank
Telescope observations to construct a spectroscopically resolved HI~21-cm map
of this entire complex at a $17.1\lesssim\log{\left({N_{\rm HI},\,1\sigma}/{\rm
cm}^{-2}\right)}\lesssim17.9$ sensitivity for a ${\rm FWHM}=20~{\rm km}\,{\rm
s}^{-1}$ line and $\Delta\theta=9.1\,{\rm arcmins}$ or $17\lesssim\Delta
d_{\theta}\lesssim30~\rm pc$ spatial resolution. We find that that Complex A is
has a Galactic standard of rest frame velocity gradient of $\Delta\rm
v_{GSR}/\Delta L=25~{\rm km}\,{\rm s}^{-1}/{\rm kpc}$ along its length, that it
is decelerating at a rate of $\langle a\rangle_{\rm GSR}=55~{\rm km}/{\rm
yr}^2$, and that it will reach the Galactic plane in $\Delta t\lesssim70~{\rm
Myrs}$ if it can survive the journey. We have identify numerous signatures of
gas disruption. The elongated and multi-core structure of Complex A indicates
that either thermodynamic instabilities or shock-cascade processes have
fragmented this stream. We find Rayleigh-Taylor fingers on the low-latitude
edge of this HVC; many have been pushed backward by ram-pressure stripping. On
the high-latitude side of the complex, Kelvin-Helmholtz instabilities have
generated two large wings that extend tangentially off Complex A. The tips of
these wings curve slightly forward in the direction of motion and have an
elevated \hi\ column density, indicating that these wings are forming
Rayleigh-Taylor globules at their tips and that this gas is becoming entangled
with unseen vortices in the surrounding coronal gas. These observations provide
new insights on the survivability of low-metallicity gas streams that are
accreting onto $L_\star$ galaxies.
