Abstract. We present the final data release of observations of λ21-cm emission from Galactic neutral hydrogen over the entire sky, merging the Leiden/Dwingeloo Survey (LDS: Hartmann & Burton 1997, Atlas of Galactic Neutral Hydrogen) of the sky north of δ = −30• with the Instituto Argentino de Radioastronomía Survey (IAR: Arnal et al. 2000, A&AS, 142, 35; and Bajaja et al. 2005, A&A, 440, 767) of the sky south of δ = −25• . The angular resolution of the combined material is HPBW ∼ 0.• 6. The LSR velocity coverage spans the interval −450 km s −1 to +400 km s −1 , at a resolution of 1.3 km s −1 . The data were corrected for stray radiation at the Institute for Radioastronomy of the University of Bonn, refining the original correction applied to the LDS. The rms brightness-temperature noise of the merged database is 0.07−0.09 K. Residual errors in the profile wings due to defects in the correction for stray radiation are for most of the data below a level of 20−40 mK. It would be necessary to construct a telescope with a main beam efficiency of η MB > ∼ 99% to achieve the same accuracy. The merged and refined material entering the LAB Survey of Galactic H  is intended to be a general resource useful to a wide range of studies of the physical and structural characteristices of the Galactic interstellar environment. The LAB Survey is the most sensitive Milky Way H  survey to date, with the most extensive coverage both spatially and kinematically.

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We suggest that the high-velocity clouds (HVCs) are large clouds, with typical diameters of 25 kpc, containing 3 ] 107 of neutral gas and 3 ] 108 of dark matter, falling onto the Local Group ; M _ M _ altogether the HVCs contain 1010 of neutral gas. Our reexamination of the Local Group hypothesis M _ for the HVCs connects their properties to the hierarchical structure formation scenario and to the gas seen in absorption toward quasars. We show that at least one HVC complex (besides the Magellanic Stream) must be extragalactic 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 108We discuss a number of other clouds that M _ . are positionally associated with the Local Group 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 evolution of the Local Group and Ðnd that material falling onto the Local Group reproduces the location of two of the three most signiÐcant groupings of clouds and the kinematics of the entire cloud ensemble (excluding the Magellanic Stream). We interpret the third grouping (the A, C, and M complexes) as the nearest HVC. It is tidally unstable and is falling onto the Galactic disk. We interpret the more distant HVCs as gas contained within dark matter "" minihalos ÏÏ moving along Ðlaments 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 quasars. 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 with the rest of the interstellar medium.We argue that there is a Galactic fountain in the Milky Way, but that the fountain does not explain the origin of the HVCs. Our analysis of the H I data leads to the detection of a vertical infall of lowvelocity gas toward the plane and implies that the H I disk is not in hydrostatic equilibrium. We suggest that the fountain is manifested mainly by relatively local neutral 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. The HVCs should have subsolar metallicities. Their Ha emission should be less 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 observations are consistent with these predictions and discuss future tests.

We explore the Magellanic Stream (MS) using a Gaussian decomposition of the HI velocity profiles in the Leiden-Argentine-Bonn (LAB) all-sky HI survey. This decomposition exposes the MS to be composed of two filaments distinct both spatially (as first pointed out by Putman et al.) and in velocity. Using the velocity coherence of the filaments, one can be traced back to its origin in what we identify as the SouthEast HI Overdensity (SEHO) of the Large Magellanic Cloud (LMC), which includes 30 Doradus. Parts of the Leading Arm (LA) can also be traced back to the SEHO in velocity and position. Therefore, at least one-half of the trailing Stream and most of the LA originates in the LMC, contrary to previous assertions that both the MS and the LA originate in the Small Magellanic Cloud (SMC) and/or in the Magellanic Bridge. The two MS filaments show strong periodic, undulating spatial and velocity patterns that we speculate are an imprint of the LMC rotation curve. If true, then the drift rate of the Stream gas away from the Magellanic Clouds is ~49 km/s and the age of the MS is ~1.74 Gyr. The Staveley-Smith et al. high-resolution HI data of the LMC show gas outflows from supergiant shells in the SEHO that seem to be creating the LA and LMC filament of the MS. Blowout of LMC gas is an effect not previously accounted for but one that probably plays an important role in creating the MS and LA.Comment: 31 pages, 26 figures, Accepted for publication in Ap

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