Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. H I observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: (1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by H I cloud complexes, tails and filaments. This suggests ongoing minor mergers and recent arrival of external gas. It may be regarded, therefore, as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M 31. (2) Considerable amounts of extra-planar H I have been found in nearby spiral galaxies. While a large fraction of this gas is undoubtedly produced by galactic fountains, it is likely that a part of it is of extragalactic origin. Also the Milky Way has extra-planar gas complexes: the Intermediate-and High-Velocity Clouds (IVCs and HVCs). be sustained. Gas infall has been proposed as the origin. (4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The new gas could be added to the halo or be deposited in the outer parts of galaxies and form reservoirs for replenishing the inner parts and feeding star formation. The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean "visible" accretion rate of cold gas in galaxies of at least 0.2 M year −1 . In order to reach the accretion rates needed to sustain the observed star formation (≈ 1 M year −1 ), additional infall of large amounts of gas from the IGM seems to be required.

Abstract. Neutral hydrogen observations with the Westerbork Synthesis Radio Telescope are presented for a sample of 73 latetype dwarf galaxies. These observations are part of the WHISP project (Westerbork H  Survey of Spiral and Irregular Galaxies). Here we present H  maps, velocity fields, global profiles and radial surface density profiles of H , as well as H  masses, H  radii and line widths. For the late-type galaxies in our sample, we find that the ratio of H  extent to optical diameter, defined as 6.4 disk scale lengths, is on average 1.8 ± 0.8, similar to that seen in spiral galaxies. Most of the dwarf galaxies in this sample are rich in H  with a typical M H i /L B of 1.5. The relative H  content M H i /L R increases towards fainter absolute magnitudes and towards fainter surface brightnesses. Dwarf galaxies with lower average H  column densities also have lower average optical surface brightnesses. We find that lopsidedness is as common among dwarf galaxies as it is in spiral galaxies. About half of the dwarf galaxies in our sample have asymmetric global profiles, a third has a lopsided H  distribution, and about half shows signs of kinematic lopsidedness.

A comprehensive analysis of 355 high‐quality Westerbork Synthesis Radio Telescope (WSRT) H i 21‐cm line maps of nearby galaxies shows that the properties and incident rate of damped Lyman α absorption systems (DLAs) observed in the spectra of high‐redshift QSOs are in good agreement with DLAs originating in gas discs of galaxies like those in the z≈ 0 population. Comparison of low‐z DLA statistics with the H i incidence rate and column density distribution f(NH i) for the local galaxy sample shows no evidence for evolution in the integral ‘cross‐section density’〈nσ〉=l−1 (l= mean free path between absorbers) below z≈ 1.5, implying that there is no need for a hidden population of galaxies or H i clouds to contribute significantly to the DLA cross‐section. Compared with z≈ 4, our data indicate evolution of a factor of 2 in the comoving density along a line of sight. We find that dN/dz(z= 0) = 0.045 ± 0.006. The idea that the local galaxy population can explain the DLAs is further strengthened by comparing the properties of DLAs and DLA galaxies with the expectations based on our analysis of local galaxies. The distribution of luminosities of DLA host galaxies, and of impact parameters between QSOs and the centres of DLA galaxies, is in good agreement with what is expected from local galaxies. Approximately 87 per cent of low‐z DLA galaxies are expected to be fainter than L*, and 37 per cent have impact parameters less than 1 arcsec at z= 0.5. The analysis shows that some host galaxies with very low impact parameters and low luminosities are expected to be missed in optical follow‐up surveys. The well‐known metallicity–luminosity relation in galaxies, in combination with metallicity gradients in galaxy discs, causes the expected median metallicity of low‐z DLAs to be low (∼1/7 solar), which is also in good agreement with observations of low‐z DLAs. We find that f(NH i) can be fitted satisfactorily with a gamma distribution, a single power law is not a good fit at the highest column densities NH i > 1021 cm−2. The vast majority (≈81 per cent) of the H i gas in the local Universe resides in column densities above the classical DLA limit (NH i > 2 × 1020 cm−2), with NH i∼ 1021 cm−2 dominating the cosmic H i mass density.

We revisit the Hi size-mass (D HI -M HI ) relation of galaxies with a sample of more than 500 nearby galaxies covering over five orders of magnitude in Hi mass and more than ten B-band magnitudes. The relation is remarkably tight with a scatter σ ∼0.06 dex, or 14%. The scatter does not change as a function of galaxy luminosity, Hi richness or morphological type. The relation is linked to the fact that dwarf and spiral galaxies have a homogenous radial profile of Hi surface density in the outer regions when the radius is normalised by D HI . The early-type disk galaxies typically have shallower Hi radial profiles, indicating a different gas accretion history. We argue that the process of atomic-to-molecular gas conversion or star formation cannot explain the tightness of the D HI -M HI relation. This simple relation puts strong constraints on simulation models for galaxy formation.

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