We present a wide (8.5 • × 6.7 • , 1050 × 825 kpc), deep (σ NHI = 10 16.8−17.5 cm −2 ) neutral hydrogen (HI) map of the M101 galaxy group. We identify two new HI sources in the group environment, one an extremely low surface brightness (and hitherto unknown) dwarf galaxy, and the other a starless HI cloud, possibly primordial in origin. Our data show that M101's extended HI envelope takes the form of a ∼ 100 kpc long tidal loop or plume of HI extending to the southwest of the galaxy. The plume has an HI mass ∼ 10 8 M and a peak column density of N HI = 5 × 10 17 cm −2 , and while it rotates with the main body of M101, it shows kinematic peculiarities suggestive of a warp or flaring out of the rotation plane of the galaxy. We also find two new HI clouds near the plume with masses ∼ 10 7 M , similar to HI clouds seen in the M81/M82 group, and likely also tidal in nature. Comparing to deep optical imaging of the M101 group, neither the plume nor the clouds have any extended optical counterparts down to a limiting surface brightness of µ B = 29.5. We also trace HI at intermediate velocities between M101 and NGC 5474, strengthening the case for a recent interaction between the two galaxies. The kinematically complex HI structure in the M101 group, coupled with the optical morphology of M101 and its companions, suggests that the group is in a dynamically active state that is likely common for galaxies in group environments.

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We present a 3 • ×3 • , 105-pointing, high-resolution neutral hydrogen (H i) mosaic of the M81 galaxy triplet, (including the main galaxies M81, M82 and NGC 3077, as well as dwarf galaxy NGC 2976) obtained with the Very Large Array (VLA) C and D arrays. This H i synthesis mosaic uniformly covers the entire area and velocity range of the triplet. The observations have a resolution of ∼ 20 or ∼ 420 pc. The data reveal many small-scale anomalous velocity features highlighting the complexity of the interacting M81 triplet. We compare our data with Green Bank Telescope (GBT) observations of the same area. This comparison provides evidence for the presence of a substantial reservoir of low-column density gas in the northern part of the triplet, probably associated with M82. Such a reservoir is not found in the southern part. We report a number of newly discovered kpc-sized low-mass H i clouds with H i masses of a few times 10 6 M . A detailed analysis of their velocity widths show that their dynamical masses are much larger than their baryonic masses, which could indicate the presence of dark matter if the clouds are rotationally supported. However, due to their spatial and kinematical association with H i tidal features, it is more likely that the velocity widths indicate tidal effects or streaming motions. We do not find any clouds that are not associated with tidal features down to an H i mass limit of a few times 10 4 M . We compare the H i column densities with resolved stellar density maps and find a star formation threshold around 3 − 6 · 10 20 cm −2 . We investigate the widths of the H i velocity profiles in the triplet and find that extreme velocity dispersions can be explained by a superposition of multiple components along the line of sight near M81 as well as winds or outflows around M82. The velocity dispersions found are high enough that these processes could explain the linewidths of Damped-Lyman-α absorbers observed at high redshift.

M82 is one of the best studied starburst galaxies in the local universe, and is consequently a benchmark for studying star formation feedback at both low and high redshift. We present new VLA H I observations that reveal the cold gas kinematics along the minor axis in unprecedented detail. This includes the detection of H I up to 10 kpc along the minor axis toward the South and beyond 5 kpc to the North. A surprising aspect of these observations is that the line-of-sight H I velocity decreases substantially from about 120 km s −1 to 50 km s −1 from 1.5 to 10 kpc off the midplane. The velocity profile is not consistent with the H I gas cooling from the hot wind. We demonstrate that the velocity decrease is substantially greater than the deceleration expected from gravitational forces alone. If the H I consists of a continuous population of cold clouds, some additional drag force must be present, and the magnitude of the drag force places a joint constraint on the ratio of the ambient medium to the typical cloud size and density. We also show that the H I kinematics are inconsistent with a simple conical outflow centered on the nucleus, but instead require the more widespread launch of the H I over the ∼ 1 kpc extent of the starburst region. Regardless of the launch mechanism for the H I gas, the observed velocity decrease along the minor axis is sufficiently great that the H I may not escape the halo of M82. We estimate the H I outflow rate is much less than 1 M ⊙ yr −1 at 10 kpc off the midplane.

Observed H i accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion occurs in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To constrain the presence and properties of such flows, we present deep H i observations obtained with the NRAO Green Bank Telescope of an area measuring 4• × 4• around NGC 2403. These observations, with a 5σ detection limit of 2.4 × 10 18 cm −2 over a 20 km s −1 linewidth, reveal a low column density, extended cloud outside the main H i disk, about 17 (∼16 kpc or ∼2 R 25 ) to the NW of the center of the galaxy. The total H i mass of the cloud is 6.3 × 10 6 M , or 0.15 percent of the total H i mass of NGC 2403. The cloud is associated with an 8 kpc anomalous-velocity H i filament in the inner disk, that was previously observed in deep VLA observations. We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigboring dwarf galaxy.