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

We discuss new, high sensitivity H I observations of the spiral galaxy NGC 2403 which show extended emission at anomalous velocities with respect to the 'cold' disk. This 'anomalous' gas component (∼ 1/10 of the total H I mass) is probably located in the region of the halo and rotates more slowly (∼ 20−50 km s −1 ) than the gas in the disk. Moreover, it shows a distortion in the velocity field that we interpret as a large-scale radial motion (10−20 km s −1 inflow) towards the centre of the galaxy. The most likely explanation for its origin and kinematics seems to be that of a galactic fountain. There is, however, a significant part of the anomalous gas which seems to be moving contrary to rotation and is

A search for H i absorption has been made in 23 radio galaxies selected from a complete sample. The observations were made with the Australia Telescope Compact Array (ATCA), with the Very Large Array (VLA) and for one galaxy with the Westerbork Synthesis Radio Telescope (WSRT). In five galaxies H i absorption was detected. We investigate how the detection rate is distributed among galaxies with different radio and optical properties. Among the Fanaroff–Riley (FR) type I radio galaxies, only one object (10 per cent of the total) was detected in H i absorption. The H i absorption in these objects is likely to come from a nuclear disc, as found for other galaxies of this type (e.g. NGC 4261 and Hydra A). The low detection rate is consistent with the hypothesis (as suggested by optical and X‐ray data) that the ‘standard’ parsec‐scale, geometrically thick torus is not required in low‐luminosity radio galaxies. This is consistent with earlier optical work. In the case of FR type II powerful radio galaxies, no H i absorption has been detected in broad‐line radio galaxies, while three out of four narrow‐line radio galaxies have been detected (the one non‐detection having quite a high upper limit). All of these are compact or small radio galaxies. To first order this is consistent with the predictions of the unified schemes, assuming that the H i absorption is caused by an obscuring torus. However, the indications of this being the only cause of the absorption are not very strong. In particular, we find that in two of the three detected objects the H i is blueshifted compared with the systemic velocity. In the third galaxy (PKS 1549–79) two redshift systems (from the optical lines) are found. The uncertainty in the systemic velocity derived from optical lines is discussed. Finally, by also considering data available in the literature, we find a tendency for radio galaxies with a strong component of young stellar population and far‐infrared emission to show H i absorption. The overall richer interstellar medium that is likely to be present in these galaxies may be a factor in producing the absorption.

Abstract. We present high-spatial resolution 21-cm H I VLA observations of the radio galaxy 3C 305 (z = 0.041). These new high-resolution data show that the ∼1000 km s −1 broad H  absorption, earlier detected in low-resolution WSRT observations, is occurring against the bright, eastern radio lobe, about 1.6 kpc from the nucleus. We use new optical spectra taken with the WHT to make a detailed comparison of the kinematics of the neutral hydrogen with that of the ionised gas. The striking similarity between the complex kinematics of the two gas phases suggests that both the ionised gas and the neutral gas are part of the same outflow. Earlier studies of the ionised gas had already found evidence for a strong interaction between the radio jet and the interstellar medium at the location of the eastern radio lobe. Our results show that the fast outflow produced by this interaction also contains a component of neutral atomic hydrogen. The most likely interpretation is that the radio jet ionises the ISM and accelerates it to the high outflow velocities observed. Our observations demonstrate that, following this strong jet-cloud interaction, not all gas clouds are destroyed and that part of the gas can cool and become neutral. The mass outflow rate measured in 3C 305 is comparable, although at the lower end of the distribution, to that found in Ultra-Luminous IR galaxies. This suggests that AGN-driven outflows, and in particular jet-driven outflows, can have a similar impact on the evolution of a galaxy as starburst-driven superwinds.

We present WSRT, VLA and VLBI observations of the H i absorption in the radio galaxy NGC 315. The main result is that two H i absorbing systems are detected against the central region. In addition to the known highly redshifted, very narrow component, we detect relatively broad (FWZI ∼ 150 km s −1 ) absorption. This broad component is redshifted by ∼80 km s −1 compared to the systemic velocity, while the narrow absorption is redshifted ∼490 km s −1 . Both H i absorption components are spatially resolved at the pcscale of the VLBI observations. The broad component shows strong gradients in density (or excitation) and velocity along the jet. We conclude that this gas is physically close to the AGN, although the nature of the gas resulting in the broad absorption is not completely clear. The possibility that it is entrained by the radio jet (and partly responsible of the deceleration of the jet) appears unlikely. Gas located in a thick circumnuclear toroidal structure, with orientation similar to the dusty, circumnuclear disk observed with HST, cannot be completely ruled out although it appears difficult to reconcile with the observed morphology and kinematics of the H i. A perhaps more likely scenario is that the gas producing the broad absorption could be (directly or indirectly) connected with the fueling of the AGN, i.e. gas that is falling into the nucleus. If this is the case, the accretion rate derived is similar (considering all uncertainties) to that found for other X-ray luminous elliptical galaxies, although lower than that derived from the radio core luminosity for NGC 315. The data also show that, in contrast to the broad component, the density distribution of the narrow component is featureless. Moreover, in the WSRT observations we do detect a small amount of H i in emission a few kpc SW of the AGN, coincident with faint optical absorption features and at velocities very similar to the narrow absorption. This suggests that the gas causing the narrow absorption is not close to the AGN and is more likely caused by clouds falling into NGC 315. The environment of NGC 315 turns out to be indeed quite gas rich since we detect five gas-rich companion galaxies in the immediate vicinity of NGC 315.

Abstract. The radio source 4C 12.50 has often been suggested to be a prime candidate for the link between ultraluminous infrared galaxies and young radio galaxies. A VLBI study of the neutral hydrogen in the nuclear regions of this object shows that most of the gas detected close to the systemic velocity is associated with an off-nuclear cloud (∼50 to 100 pc from the radio core) with a column density of ∼10 22 T spin /(100 K) cm −2 and an H I mass of a few times 10 5 to 10 6 M . We consider a number of possibilities to explain the results. In particular, we discuss the possibility that this cloud indicates the presence of a rich and clumpy interstellar medium in the centre, likely left over from the merger that triggered the activity and that this medium influences the growth of the radio source. The location of the cloud -at the edge of the northern radio jet/lobe -suggests that the radio jet might be interacting with a gas cloud. This interaction could be responsible for bending the young radio jet. The velocity profile of the gas is relatively broad (∼150 km s −1 ) and we interpret this as kinematical evidence for interaction of the radio plasma with the cloud. We also consider the model where the cloud is part of a broader circumnuclear structure. Only a limited region of this structure would have sufficient background radio brightness and large enough column depth in neutral gas to obtain detectable H I absorption against the counterjet. The VLBI study of the neutral hydrogen in 4C 12.50 suggests that H I detected near the systemic velocity (as it is often the case in radio galaxies) may not necessarily be connected with a circumnuclear disk or torus (as is very often assumed) but instead could be a tracer of the large-scale medium that surrounds the active nucleus and that may influence the growth of the young radio source.

We present results of a study of neutral hydrogen (H I) in a complete sample of nearby non-cluster radio galaxies. We find that radio galaxies with large amounts of extended H I (M H I 10 9 M ) all have a compact radio source. The host galaxies of the more extended radio sources, all of Fanaroff & Riley type-I, do not contain these amounts of H I. We discuss several possible explanations for this segregation. The large-scale H I is mainly distributed in disk-and ring-like structures with sizes up to 190 kpc and masses up to 2 × 10 10 M . The formation of these structures could be related to past merger events, although in some cases it may also be consistent with a cold-accretion scenario.

We have mapped NGC 3718, a nearby bright galaxy in a loose group, and its companion NGC 3729 in the 21 cm line of neutral hydrogen. NGC 3718 is a strikingly unusual galaxy with a strong straight dust lane across the center, peculiar diffuse spiral arms, and an extended disk of neutral hydrogen. Earlier work showed the gas disk to be strongly twisted, warping through edge-on where we see the straight dust lane; stars formed in this gas appear to make up the 'spiral arms'. Our improved maps show a twisted but bisymmetric disk of gas extending to 7 or 35 kpc, where the orbital period is roughly 1 Gyr. It is surrounded by fragmentary spiral features, and a streamer of gas extending to a cloud lying 12 or 60 kpc to the north. We use inspector, a task in gipsy, to fit a tilted-ring model interactively to slices through the Hi data cube. The apparent major axis swings through 100 • from the innermost gas orbits at 30 radius to the outer edge. When viewed in the reference frame of the galaxy's stellar disk, the innermost gas orbits are nearly polar, while the outer rings of gas are tilted at 30 • -40 • . The line of nodes, where the gas orbits pass through the plane of the stellar disk, twists by roughly 90 • about the pole. We do not see gas orbiting in the plane of the stellar disk. If we assume that the galaxy's dark halo shares the same midplane, then the observed twist can be explained by differential precession in a dynamical model in which the dark halo is fairly round. The run of tilt with radius is close to what is required for the warped gas disk to precess rigidly in the galaxy's gravitational field without changing its shape. This fact probably accounts for the longevity of the twisted structure.