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

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.

17 pages, 8 figures. Accepted for publication in the Astrophysical Journal

References in corpus (12)

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• Metallicity and Ionization in High Velocity Cloud Complex C
• X-ray and UV spectroscopy of Galactic diffuse hot gas along the LMC X--3 sight line
• Ongoing Galactic Accretion: Simulations and Observations of Condensed Gas in Hot Halos
• Xmm-Newton Observations of the Diffuse X-ray Background
• Ionized Gas in the Smith Cloud
• An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
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• Detection of a large fraction of atomic gas not associated with star-forming material in M17 SW
• Si iv Column Densities Predicted from Non-Equilibrium Ionization Simulations of Turbulent Mixing Layers and High-Velocity Clouds
• Numerical Simulations of a Shock Interacting with Multiple Magnetized Clouds
• The Anatomy of a Turbulent Radiative Mixing Layer: Insights from an Analytic Model with Turbulent Conduction and Viscosity
• In Search of an Interface between Warm and Hot Gas within the Local Bubble