Physical effects on compact high-velocity clouds in the circumgalactic medium

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Bastian Sander, Gerhard Hensler

We numerically investigate the evolution of compact high-velocity clouds (CHVCs) passing through a hot, tenuous gas representing the highly-ionized circumgalactic medium (CGM) by applying the adaptive-mesh refinement code FLASH.

The model clouds start from both hydrostatic and thermal equilibrium and are in pressure balance with the CGM. Here, we present 14 models, divided into two mass categories and two metallicities each and different velocities. We allow for self-gravity and thermal conduction or not. All models experience mass diffusion, radiative cooling, and external heating leading to dissociation and ionization.

Our main findings are: 1) self-gravity stabilizes clouds against Rayleigh-Taylor instability which are disrupted within 10 sound-crossing times without; 2) clouds can develop Jeans-instable regions internally even though they are initially below Jeans mass; 3) all clouds lose mass by ram pressure and Kelvin-Helmholtz instability; 4) thermal conduction substantially lowers mass-loss rates, by this, extending the clouds' lifetimes, particularly, more than doubling the lifetime of low-mass clouds; 5) thermal conduction leads to continuous, filamentary stripping, while the removed gas is heated up quickly and mixes efficiently with the ambient CGM; 6) without thermal conduction the removed gas consists of dense, cool, clumpy fragments; 7) thermal conduction might prevent CHVCs from forming stars; 8) clouds decelerated by means of drag from the ambient CGM form head-tail shapes and collapse after they reach velocities characteristic for intermediate-velocity clouds.

Conclusively, only sophisticated modelling of CHVCs as non-homogeneous and non-isothermal clouds with thermal conduction and self-gravity explains observed morphologies and naturally leads to the suppression of star formation.

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