Simulations of High-Velocity Clouds. II. Ablation from High-Velocity Clouds as a Source of Low-Velocity High Ions

TL;DR

This study uses hydrodynamic simulations to investigate how ablated material from high-velocity clouds contributes to low-velocity high ions like O VI in the Galactic halo, highlighting its significance for understanding halo ionization.

Contribution

It provides the first detailed simulations of ablation from high-velocity clouds and quantifies its role in producing low-velocity high ions in the Galactic halo.

Findings

Ablated HVC material accounts for at least 1/3 of observed O VI in the halo.

Ablation is unlikely to be the main source of C IV due to photoionization effects.

A composite model including HVCs, supernova remnants, and photoionization matches observed O VI and C IV, but not all N V.

Abstract

In order to determine if the material ablated from high-velocity clouds (HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI) such as those found in the Galactic halo, we simulate the hydrodynamics of the gas and the time-dependent ionization evolution of its carbon, nitrogen, and oxygen ions. Our suite of simulations examines the ablation of warm material from clouds of various sizes, densities, and velocities as they pass through the hot Galactic halo. The ablated material mixes with the environmental gas, producing an intermediate-temperature mixture that is rich in high ions and that slows to the speed of the surrounding gas. We find that the slow mixed material is a significant source of the low-velocity O VI that is observed in the halo, as it can account for at least ~1/3 of the observed O VI column density. Hence, any complete model of the high ions in the halo should include the contribution to the O VI from ablated HVC material. However, such material is unlikely to be a major source of the observed C IV, presumably because the observed C IV is affected by photoionization, which our models do not include. We discuss a composite model that includes contributions from HVCs, supernova remnants, a cooling Galactic fountain, and photoionization by an external radiation field. By design, this model matches the observed O VI column density. This model can also account for most or all of the observed C IV, but only half of the observed N V.