The space density of primordial gas clouds near galaxies and groups and their relation to galactic high-velocity clouds

The Arecibo H I Strip Survey probed the halos of similar to 300 cataloged g alaxies and the environments of similar to 14 groups with sensitivity to ne utral hydrogen masses greater than or equal to 10(7) M-circle dot. The surv ey detected no objects with properties resembling the high-velocity clouds (HVCs) associated with the Milky Way or Local Group. If the HVCs were typic ally M-HI = 10(7.5) M-circle dot objects distributed throughout groups and galaxy halos at distances of similar to 1 Mpc, the survey should have made similar to 70 HVC detections in groups and similar to 250 detections around galaxies. The null detection implies that HVCs are deployed at typical dis tances of less than or equal to 200 kpc from the galaxies or group barycent ers. If the clouds are in virial equilibrium, their average dark matter fra ction must be 98% or higher.