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.

In this data paper we present the results of an extensive 21 cm-line synthesis imaging survey of 43 spiral galaxies in the nearby Ursa Major cluster using the Westerbork Synthesis Radio Telescope. Detailed kinematic information in the form of position-velocity diagrams and rotation curves is presented in an atlas together with HI channel maps, 21 cm continuum maps, global HI profiles, radial HI surface density profiles, integrated HI column density maps, and HI velocity fields. The relation between the corrected global HI linewidth and the rotational velocities Vmax and V flat as derived from the rotation curves is investigated. Inclination angles obtained from the optical axis ratios are compared to those derived from the inclined HI disks and the HI velocity fields. The galaxies were not selected on the basis of their HI content but solely on the basis of their cluster membership and inclination which should be suitable for a kinematic analysis. The observed galaxies provide a well-defined, volume limited and equidistant sample, useful to investigate in detail the statistical properties of the Tully-Fisher relation and the dark matter halos around them.

We present H i observations of the edge-on galaxy NGC 891. These are among the deepest ever performed on an external galaxy. They reveal a huge gaseous halo, much more extended than seen previously and containing almost 30% of the H i. This H i halo shows structures on various scales. On one side, there is a filament extending (in projection) up to 22 kpc vertically from the disk. Small (M H i k 10 6 M ) halo clouds, some with forbidden (apparently counterrotating) velocities, are also detected. The overall kinematics of the halo gas is characterized by differential rotation lagging with respect to that of the disk. The lag, more pronounced at small radii, increases with height from the plane. There is evidence that a significant fraction of the halo is due to a galactic fountain. Accretion from intergalactic space may also play a role in building up the halo and providing the low angular momentum material needed to account for the observed rotation lag. The long H i filament and the counterrotating clouds may be direct evidence of such accretion.

High sensitivity H I observations of the nearby spiral galaxy NGC 2403 obtained with the VLA are presented and discussed. The properties of the extended, differentially rotating H I layer with its H I holes, spiral structure and outer warp are described. In addition, these new data reveal the presence of a faint, extended and kinematically anomalous component. This shows up in the H I line profiles as extended wings of emission towards the systemic velocity. In the central regions these wings are very broad (up to 150 km s −1 ) and indicate large deviations from circular motion. We have separated the anomalous gas component from the cold

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.

We present rotation curves derived from H i observations for a sample of 62 galaxies that have been observed as part of the Westerbork H i Survey of Spiral and Irregular Galaxies (WHISP) project. These rotation curves have been derived by interactively fitting model data cubes to the observed cubes. This procedure takes the rotation curve shape, the H i distribution, the inclination, and the size of the beam into account, and makes it possible to correct for the effects of beam smearing. A comparison with higher spatial resolution Hα rotation curves available in the literature shows that there is general agreement between the two. The late-type dwarf galaxies in our sample have rotation-curve shapes that are similar to those of late-type spiral galaxies, in the sense that their rotation curves, when expressed in units of disk scale lengths, rise as steeply in the inner parts and start to flatten at two disk scale lengths. None of the galaxies in our sample have solid-body rotation curves that extend beyond three disk scale lengths. The logarithmic slopes between two disk scale lengths and the last measured point on the rotation curve is similar between late-type dwarf and spiral galaxies. Thus, whether the flat part of the rotation curve is reached or not seems to depend more on the extent of the rotation curve than on its amplitude. We also find that the outer rotation curve shape does not strongly depend on luminosity, at least for galaxies fainter than M R ∼ −19. We find that in spiral galaxies and even in the central regions of late-type dwarf galaxies, the shape of the central distribution of light and the inner rise of the rotation curve are related. This implies that galaxies with stronger central concentrations of light also have higher central mass densities, and it suggests that the luminous mass dominates the gravitational potential in the central regions, even in low surface brightness dwarf galaxies.

We present a study of the distribution and kinematics of the neutral gas in the low-inclination Scd galaxy NGC 6946. The galaxy has been observed for 192 h at 21-cm with the Westerbork Synthesis Radio Telescope. These are among the deepest observations ever obtained for a nearby galaxy. We detect widespread high-velocity H I (up to about 100 km s −1 ) and find 121 H I holes, most of which are located in the inner regions where the gas density and the star formation rate are higher. Much of the high-velocity gas appears to be related to star formation and to be, in some cases, associated with H I holes. The overall kinematics of the high-velocity gas is characterized by a slower rotation as compared with the regular disk rotation. We conclude that the high-velocity gas in NGC 6946 is extra-planar and has the same properties as the gaseous halos observed in other spiral galaxies including the Milky Way. Stellar feedback (galactic fountain) is probably at the origin of most of the high-velocity gas and of the H I holes. There are also indications, especially in the outer regions, -an extended H I plume, velocity anomalies, sharp edges, and large-scale asymmmetries -pointing to tidal encounters and recent gas accretion.

No abstract