Compact high-velocity clouds ( CHVCs ) are the most distant of the HVCs in the Local Group model and , at d \sim 1 Mpc , they have HI volume densities of \sim 3 \times 10 ^ { -4 } cm ^ { -3 } .
Clouds with these volume densities and the observed column densities { N _ { HI } } \sim 10 ^ { 19 } cm ^ { -2 } will be largely ionized , even if exposed only to the extragalactic ionizing radiation field .
Here we examine the implications of this process for models of CHVCs .
We have modeled the ionization structure of spherical clouds ( with and without dark matter halos ) for a large range of densities and sizes , appropriate to CHVCs over the range of suggested distances , exposed to an extragalactic ionizing photon flux \phi _ { i } \sim 10 ^ { 4 } phot cm ^ { -2 } s ^ { -1 } .
Constant-density cloud models in which the CHVCs are at Local Group distances have total ( ionized plus neutral ) gas masses \sim 20 - 30 times larger than the neutral gas masses , implying that the gas mass alone of the observed population of CHVCs is \sim 4 \times 10 ^ { 10 } M _ { \odot } .
With a realistic ( 10:1 ) dark matter to gas mass ratio , the total mass in such CHVCs is a significant fraction of the dynamical mass of the Local Group , and their line widths would greatly exceed the observed \Delta V .
Self-consistent models of gas in dark matter halos fare even more poorly ; they must lie within approximately 200 kpc of the Galaxy , and ( for a given distance ) are much more massive than the corresponding uniform density models .
We also show that exponential neutral hydrogen column density profiles are a natural consequence of an external source of ionizing photons , and argue that these profiles can not be used to derive model-independent distances to the CHVCs .
These results argue strongly that the CHVCs are not cosmological objects , and are instead associated with the Galactic halo .