Exploring Hydrodynamic Instabilities along the Infalling High-Velocity Cloud Complex A

Complex A is a high-velocity cloud (HVC) that is traversing through the Galactic halo toward the Milky Way’s disk. We used Green Bank Telescope observations to construct a spectroscopically resolved HI 21 cm map of this entire complex at roughly log(N_HI/cm^-2) = 17.9 (1-sigma) sensitivity and 25 pc spatial resolution. We find that that Complex A will reach the Galactic plane in roughly 70 Myr if it can survive its journey. We have identified numerous signatures of gas disruption, including an elongated and multi-core structure that are characteristic of fragmentation due to either thermodynamic instabilities or shock-cascade processes. We also 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; these wings are forming Rayleigh-Taylor globules at their tips. These observations provide new insights on the survivability of low-metallicity gas streams that are accreting onto L-star galaxies.