2020ApJ...902..154B

Complex A is a high-velocity cloud (HVC) 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 H I 21 cm map of this entire complex at a 17.1≲log(N_HI,1σ/cm^–2)≲17.9 sensitivity for a FWHM=20kms^–1 line and Δθ = 9.'1 or 17 <= Δd_θ <= 30 pc spatial resolution. We find that Complex A has a Galactic standard of rest frame velocity gradient of Δv_GSR/ΔL=25kms^–1kpc^–1 along its length, that it is decelerating at a rate of <a>_GSR=55kmyr^–2, and that it will reach the Galactic plane in Δt <= 70 Myr if it can survive the journey. We have identified 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 H I 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_* galaxies.