Exploring Hydrodynamic Instabilities along the Infalling High-velocity Cloud Complex A
Abstract
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 $17.1\lesssim \mathrm{log}({N}_{{\rm{H}}{\rm\small{I}},1\sigma }/{\mathrm{cm}}^{-2})\lesssim 17.9$ sensitivity for a $\mathrm{FWHM}=20\,\mathrm{km}\,{{\rm{s}}}^{-1}$ line and Δθ = 9'1 or 17 ≲ Δdθ ≲ 30 pc spatial resolution. We find that Complex A has a Galactic standard of rest frame velocity gradient of ${\rm{\Delta }}{v}_{\mathrm{GSR}}/{\rm{\Delta }}L=25\,\mathrm{km}\,{{\rm{s}}}^{-1}\,{\mathrm{kpc}}^{-1}$ along its length, that it is decelerating at a rate of $\langle a{\rangle }_{\mathrm{GSR}}=55\,\mathrm{km}\,{\mathrm{yr}}^{-2}$ , 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2020
- DOI:
- 10.3847/1538-4357/abb376
- arXiv:
- arXiv:2101.11746
- Bibcode:
- 2020ApJ...902..154B
- Keywords:
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- Galaxy evolution;
- Circumgalactic medium;
- Hydrodynamics;
- High-velocity clouds;
- 594;
- 1879;
- 1963;
- 735;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Figure 5 of the paper is a video in the published that can be found here: https://www.youtube.com/watch?v=YSbJetacspo