Simulations of High-Velocity Clouds. I. Hydrodynamics and High-Velocity High Ions

TL;DR

This paper uses hydrodynamic simulations to study high-velocity clouds in the Galactic halo, predicting ionization states and column densities that match observations, and exploring cloud destruction over time.

Contribution

It introduces detailed hydrodynamic models with non-equilibrium ionization tracking to analyze cloud evolution and ion signatures, providing insights into the origin and fate of high-velocity clouds.

Findings

Predicted ion column densities match observations of Complex C.

Cloud interactions with the Galactic environment last tens to hundreds of megayears.

Smaller clouds lose most of their mass within 60 Myr, larger clouds remain largely intact over 240 Myr.

Abstract

We present hydrodynamic simulations of high-velocity clouds (HVCs) traveling through the hot, tenuous medium in the Galactic halo. A suite of models was created using the FLASH hydrodynamics code, sampling various cloud sizes, densities, and velocities. In all cases, the cloud-halo interaction ablates material from the clouds. The ablated material falls behind the clouds, where it mixes with the ambient medium to produce intermediate-temperature gas, some of which radiatively cools to less than 10,000 K. Using a non-equilibrium ionization (NEI) algorithm, we track the ionization levels of carbon, nitrogen, and oxygen in the gas throughout the simulation period. We present observation-related predictions, including the expected H I and high ion (C IV, N V, and O VI) column densities on sight lines through the clouds as functions of evolutionary time and off-center distance. The predicted column densities overlap those observed for Complex C. The observations are best matched by clouds that have interacted with the Galactic environment for tens to hundreds of megayears. Given the large distances across which the clouds would travel during such time, our results are consistent with Complex C having an extragalactic origin. The destruction of HVCs is also of interest; the smallest cloud (initial mass \approx 120 Msun) lost most of its mass during the simulation period (60 Myr), while the largest cloud (initial mass \approx 4e5 Msun) remained largely intact, although deformed, during its simulation period (240 Myr).