High-velocity clouds: Building blocks of the local group

We suggest that the high-velocity clouds (HVCs) are large clouds, with typi cal diameters of 25 kpc, containing 3 x 10(7) M. of neutral gas and 3 x 10( 8) M. of dark matter, falling onto the Local Group; altogether the HVCs con tain 10(10) M. of neutral gas. Our reexamination of the Local Group hypothe sis for the HVCs connects their properties to the hierarchical structure fo rmation scenario and to the gas seen in absorption toward quasars. We show that at least one HVC complex (besides the Magellanic Stream) must be extra galactic at a distance of more than 40 kpc from the Galactic center, with a diameter greater than 20 kpc and a mass of more than 10(8) M.. We discuss a number of other clouds that are positionally associated with the Local Gr oup galaxies, and we show that the entire ensemble of HVCs is inconsistent with a Galactic origin. The observed kinematics imply rather that the HVCs are falling toward the Local Group barycenter. We simulate the dynamical ev olution of the Local Group and find that material falling onto the Local Gr oup reproduces the location of two of the three most significant groupings of clouds and the kinematics of the entire cloud ensemble (excluding the Ma gellanic Stream). We interpret the third grouping (the A, C, and M complexe s) as the nearest HVC. It is tidally unstable and is falling onto the Galac tic disk. We interpret the more distant HVCs as gas contained within dark m atter "minihalos" moving along filaments toward the Local Group. Most poor galaxy groups should contain similar H I clouds bound to the group at large distances from the individual galaxies. We suggest that the HVCs are local analogs of the Lyman limit absorbing clouds observed against distant quasa rs. Our picture implies that the chemical evolution of the Galactic disk is governed by episodic infall of metal-poor HVC gas that only slowly mixes w ith the rest of the interstellar medium. We argue that there is a Galactic fountain in the Milky Way, but that the f ountain does not explain the origin of the HVCs. Our analysis of the H I da ta leads to the detection of a vertical infall of low-velocity gas toward t he plane and implies that the H I disk is not in hydrostatic equilibrium. W e suggest that the fountain is manifested mainly by relatively local neutra l gas with characteristic velocities of 6 km s(-1) rather than 100 km s(-1) . The Local Group infall hypothesis makes a number of testable predictions. T he HVCs should have subsolar metallicities. Their Ha emission should be les s than that seen from the Magellanic Stream. The clouds should not be seen in absorption against nearby stars. The clouds should be detectable in both emission and absorption around other galaxy groups. We show that current o bservations are consistent with these predictions and discuss future tests.