Where Are the High-Velocity Clouds?

Recent observations of high-velocity clouds (HVCs) have revealed compression fronts and tail-shaped features of H I, suggesting that they are interacting with an external medium. We perform three-dimensional hydrodynamical simulations of HVCs moving through a diffuse hot gaseous component, investigating the behavior of both extragalactic dark matter-dominated HVCs and nearby pure gas clouds that may be accreting onto the Galactic disk via a Galactic fountain. Both scenarios can give rise to similar features as observed if the external medium has a density greater than 10-4 cm-3. Observations suggest that this may be too high for a hot ionized halo or intergalactic gas and supports the Galactic fountain origin, a model that can also account for the high fraction of HVCs with tails and asymmetrical morphologies.

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