The Space Density of Primordial Gas Clouds near Galaxies and Groups and their Relation to Galactic HVCs

The Arecibo H i Strip Survey probed the halos of ∼300 cataloged galaxies and the environments of ∼14 groups with sensitivity to neutral hydrogen masses ≥ 10 M⊙. The survey detected no objects with properties resembling the High Velocity Clouds (HVCs) associated with the Milky Way or Local Group. If the HVCs were typically MHI = 10 7.5 M⊙ objects distributed throughout groups and galaxy halos at distances of ∼1 Mpc, the survey should have made ∼70 HVC detections in groups and ∼250 detections around galaxies. The null detection implies that HVCs are deployed at typical distances of ≤200 kpc from the galaxies or group barycenters. If the clouds are in virial equilibrium, their average dark matter fraction must be 98% or higher. Subject headings: Local Group — intergalactic medium — galaxies: ISM – galaxies: luminosity function, mass function — radio lines: ISM