Metal-Ion Absorption in Conductively Evaporating Clouds

Abstract

We present computations of the ionization structure and metal-absorption properties of thermally conductive interface layers that surround evaporating warm spherical clouds, embedded in a hot medium. We rely on the analytical formalism of Dalton & Balbus to calculate the temperature profile in the evaporating gas, and explicitly solve the time-dependent ionization equations for H, He, C, N, O, Si, and S in the interface. We include photoionization by an external field. We estimate how departures from equilibrium ionization affect the resonance-line cooling efficiencies in the evaporating gas, and determine the conditions for which radiative losses may be neglected in the solution for the evaporation dynamics and temperature profile. Our results indicate that non-equilibrium cooling significantly increases the value of the saturation parameter at which radiative losses begin to affect the flow dynamics. As applications we calculate the ion fractions and projected column densities arising in the evaporating layers surrounding dwarf-galaxy-scale objects that are also photoionized by metagalactic radiation. We compare our results to the UV metal-absorption column densities observed in local highly-ionized metal-absorbers, located in the Galactic corona or intergalactic medium. Conductive interfaces significantly enhance the formation of high-ions such as C^3+, N^4+, and O^5+ relative to purely photoionized clouds, especially for clouds embedded in a high-pressure corona. However, the enhanced columns are still too low to account for the O VI columns (~1e14 cm^-2) observed in the local high-velocity absorbers. We find that O VI columns larger than ~1e13 cm^-2 cannot be produced in evaporating clouds. Our results do support the conclusion of Savage & Lehner, that absorption due to evaporating O VI likely occurs in the local interstellar medium, with characteristic columns of ~1e13 cm^-2.

Accepted for Publication in ApJ

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• Simulations of radiative turbulent mixing layers
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• Simulations of High-Velocity Clouds. I. Hydrodynamics and High-Velocity High Ions
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• How Do Supernovae Impact the Circumgalactic Medium? I. Large-Scale Fountains Around a Milky Way-Like Galaxy
• HST/COS Observations of Ionized Gas Accretion at the Disk-halo Interface of M33
• Fundamental Properties of the Highly Ionized Plasmas in the Milky Way
• Cos observations of metal line and broad lyman alpha absorption in the multi-phase o vi and ne viii system toward he 02226-4110
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• The Nature of Ionized Gas in the Milky Way Galactic Fountain
• The CGM${}^2$ Survey: Circumgalactic O VI from dwarf to massive star-forming galaxies
• Gravitational Quenching by Clumpy Accretion in Cool Core Clusters: Convective Dynamical Response to Overheating
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• Radiative turbulent mixing layers at high Mach numbers
• A Model for Line Absorption and Emission from Turbulent Mixing Layers
• The Surprisingly Constant Strength of O VI Absorbers over Cosmic Time
• Kinematics of the Magellanic Stream and Implications for its Ionization
• Exploring the Milky Way Circumgalactic Medium in a Cosmological Context with a Semi-Analytic Model
• Mass, Morphing, Metallicities: The Evolution of Infalling High Velocity Clouds
• Discovery of Magellanic Stellar Debris in the H3 Survey
• The warm Gaseous Disk and the Anisotropic Circumgalactic Medium of the Milky Way
• The Warm Gas in the MW: A Kinematical Model
• Hunting for Metals Using XQ-100 Legacy Survey Composite Spectra
• A hot gaseous galaxy halo candidate with Mg X absorption
• Probing the Structure and Kinematics of the transition Layer between the Magellanic Stream and the Halo in HI
• High-ion absorption in the proximate damped Lyman-alpha system toward Q0841+129
• Simulations of Weakly Magnetized Turbulent Mixing Layers
• A High Signal-to-Noise HST Spectrum Toward J1009+0713: Precise Absorption Measurements in the CGM of Two Galaxies