We have detected absorption lines from the high-velocity cloud (HVC) complex WB in the spectrum of the star HE 1048+0231. This detection sets an upper limit on the distance to the cloud of 8.8 kpc. Nondetection (at greater than 3 σ confidence) in the star HE 1138-1303 at 7.7 ± 0.2 kpc sets a probable lower limit. The equivalent width of the Ca II K line due to the HVC [Wλ(Ca II K) = 114.6 ± 4.4 mÅ] corresponds to a column density of N(Ca II) = (1.32 ± 0.05) × 1012 cm-2. Using an H I spectrum from the Leiden/Argentine/Bonn survey, we calculate N(Ca II)/N(H I) = (81 ± 16) × 10-9. These distance limits imply an H I mass limit of 3.8 × 105 M☉ < MH I < 4.9 × 105 M☉. The upper distance limit imposed by these observations shows that this HVC complex has a probable Galactic or circumgalactic origin. Future metallicity measurements will be able to confirm or refute this interpretation.

The relationship between gas-rich galaxies and Lyα absorbers is addressed in this paper in the context of the baryonic content of galaxy halos. Deep Arecibo H I observations are presented of two gas-rich spiral galaxies within 125 h kpc projected distance of a Lyα absorber at a similar velocity. The galaxies investigated are close to edge-on, and the absorbers lie almost along their major axes, allowing for a comparison of the Lyα absorber velocities with galactic rotation. This comparison is used to examine whether the absorbers are diffuse gas rotating with the galaxies' halos, outflow material from the galaxies, or intergalactic gas in the low-redshift cosmic web. The results indicate that if the gas resides in the galaxies' halos, it is not rotating with the system and is possibly counterrotating. In addition, simple geometry indicates that the gas was not ejected from the galaxies, and there are no gas-rich satellites detected down to 3.6-7.5 × 106 M⊙ or remnants of satellites to 5-6 × 1018 cm-2. The gas could potentially be infalling from large radii, but the velocities and distances are rather high compared to the high-velocity clouds around the Milky Way. The most likely explanation is that the galaxies and absorbers are not directly associated, despite the vicinity of the spiral galaxies to the absorbers (58-77 h kpc from the H I edge). The spiral galaxies reside in a filament of intergalactic gas, and the gas detected by the absorber has not yet come into equilibrium with the galaxy.

We report the results of a radio monitoring program conducted at the Australia Telescope Compact Array to search for quiescent and flaring emission from seven nearby Southern late-type M and L dwarfs. Two late-type M dwarfs, the M7 V LHS 3003 and the M8 V DENIS 1048-3956, were detected in quiescent emission at 4.80 GHz. The observed emission is consistent with optically thin gyrosynchrotron emission from mildly relativistic (~1-10 keV) electrons with source densities ne ≲ 109 cm-3 in B ≳ 10 G magnetic fields. DENIS 1048-3956 was also detected in two spectacular, short-lived flares, one at 4.80 GHz (peak fν = 6.0 ± 0.8 mJy) and one at 8.64 GHz (peak fν = 29.6 ± 1.0 mJy) approximately 10 minutes later. The high brightness temperature (TB ≳ 1013 K), short emission period (~4-5 minutes), high circular polarization (~100%), and apparently narrow spectral bandwidth of these events imply a coherent emission process in a region of high electron density (ne ~ 1011-1012 cm-3) and magnetic field strength (B ~ 1 kG). If the two flare events are related, the apparent frequency drift in the emission suggests that the emitting source either moved into regions of higher electron or magnetic flux density or was compressed, e.g., by twisting field lines or gas motions. This emission may be related to a recent optical flare from this source that exhibited indications of chromospheric mass motion. The quiescent fluxes from the radio-emitting M dwarfs are too bright to support the Güdel-Benz empirical radio/X-ray relations, confirming a trend previously noted by Berger et al. The violation of these relations is symptomatic of a divergence in magnetic emission trends at and beyond spectral type M7/M8, where relative X-ray and Hα emission drops precipitously while relative radio emission appears to remain constant or possibly increases. With an apparent decline in chromospheric/coronal heating, the origin of hot coronal plasmas around ultracool dwarfs remains uncertain, although external sources, such as accretion from a residual disk or tidally distorted companions, remain possibilities worth exploring.

We present the first fully and uniformly sampled, spatially complete H. survey of the entire Magellanic System with high velocity resolution (Deltav = 1.0 km s(-1)), performed with the Parkes Telescope(star). Approximately 24 percent of the southern sky was covered by this survey on a approximate to5' grid with an angular resolution of HPBW = 14.'1. A fully automated data-reduction scheme was developed for this survey to handle the large number of H I spectra (1.5 x 10(6)). The individual Hanning smoothed and polarization averaged spectra have an rms brightness temperature noise of sigma = 0.12 K. The final data-cubes have an rms noise of sigma(rms) approximate to 0.05 K and an effective angular resolution of approximate to16'. In this paper we describe the survey parameters, the data-reduction and the general distribution of the H I gas.The Large Magellanic Cloud ( LMC) and the Small Magellanic Cloud (SMC) are associated with huge gaseous features - the Magellanic Bridge, the Interface Region, the Magellanic Stream, and the Leading Arm - with a total H. mass of M( H.) = 4.87 x 10(8) M-. [d/55 kpc](2), if all H I gas is at the same distance of 55 kpc. Approximately two thirds of this H. gas is located close to the Magellanic Clouds ( Magellanic Bridge and Interface Region), and 25% of the H I gas is associated with the Magellanic Stream. The Leading Arm has a four times lower H. mass than the Magellanic Stream, corresponding to 6% of the total H. mass of the gaseous features.We have analyzed the velocity field of the Magellanic Clouds and their neighborhood introducing a LMC-standard-of-rest frame. The H. in the Magellanic Bridge shows low velocities relative to the Magellanic Clouds suggesting an almost parallel motion, while the gas in the Interface Region has significantly higher relative velocities indicating that this gas is leaving the Magellanic Bridge building up a new section of theMagellanic Stream. The Leading Arm is connected to theMagellanic Bridge close to an extended arm of the LMC. The clouds in the Magellanic Stream and the Leading Arm show significant differences, both in the column density distribution and in the shapes of the line profiles. The H. gas in the Magellanic Stream is more smoothly distributed than the gas in the Leading Arm. These morphological differences can be explained if the Leading Arm is at considerably lower z-heights and embedded in a higher pressure ambient medium.

We present the largest catalogue to date of optical counterparts for H I radio-selected galaxies, HOPCAT. Of the 4315 H I radio-detected sources from the H I Parkes All Sky Survey (HIPASS) catalogue, we find optical counterparts for 3618 (84 per cent) galaxies. Of these, 1798 (42 per cent) have confirmed optical velocities and 848 (20 per cent) are single matches without confirmed velocities. Some galaxy matches are members of galaxy groups. From these multiple galaxy matches, 714 (16 per cent) have confirmed optical velocities and a further 258 (6 per cent) galaxies are without confirmed velocities. For 481 (11 per cent), multiple galaxies are present but no single optical counterpart can be chosen and 216 (5 per cent) have no obvious optical galaxy present. Most of these 'blank fields' are in crowded fields along the Galactic plane or have high extinctions.Isolated 'dark galaxy' candidates are investigated using an extinction cut of A(Bj) < 1 mag and the blank-fields category. Of the 3692 galaxies with an A(Bj) extinction < 1 mag, only 13 are also blank fields. Of these, 12 are eliminated either with follow-up Parkes observations or are in crowded fields. The remaining one has a low surface brightness optical counterpart. Hence, no isolated optically dark galaxies have been found within the limits of the HIPASS survey.

We have discovered a number of very small isolated HII regions 20-30 kpc from their nearest galaxy. The HII regions appear as tiny emission line dots (ELdots) in narrow band images obtained by the NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG). We have spectroscopic confirmation of 5 isolated HII regions in 3 systems. The H-alpha luminosities of the HII regions are equivalent to the ionizing flux of only 1 large or a few small OB stars each. These stars appear to have formed in situ and represent atypical star formation in the low density environment of galaxy outskirts. In situ star formation in the intergalactic medium offers an alternative to galactic wind models to explain metal enrichment. In interacting systems (2 out of 3), isolated HII regions could be a starting point for tidal dwarf galaxies.

The H I Parkes All-Sky Survey (HIPASS) catalogue forms the largest uniform catalogue of H I sources compiled to date, with 4315 sources identified purely by their H I content. The catalogue data comprise the southern region delta < + 2 degrees of HIPASS, the first blind H I survey to cover the entire southern sky. The rms noise for this survey is 13 mJy beam(-1) and the velocity range is -1280 to 12 700 km s(-1). Data search, verification and parametrization methods are discussed along with a description of measured quantities. Full catalogue data are made available to the astronomical community including positions, velocities, velocity widths, integrated fluxes and peak flux densities. Also available are on-sky moment maps, position-velocity moment maps and spectra of catalogue sources. A number of local large-scale features are observed in the space distribution of sources, including the super-Galactic plane and the Local Void. Notably, large-scale structure is seen at low Galactic latitudes, a region normally obscured at optical wavelengths.

A number of very small isolated HII regions have been discovered at projected distances up to 30 kpc from their nearest galaxy. These HII regions appear as tiny emission line objects in narrow band images obtained by the NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG). We present spectroscopic confirmation of four isolated HII regions in two systems, both systems have tidal HI features. The results are consistent with stars forming in interactive debris due to cloud-cloud collisions. The H-alpha luminosities of the isolated HII regions are equivalent to the ionizing flux of only a few O stars each. They are most likely ionized by stars formed in situ, and represent atypical star formation in the low density environment of the outer parts of galaxies. A small but finite intergalactic star formation rate will enrich and ionize the surrounding medium. In one system, NGC 1533, we calculate a star formation rate of 1.5e-3 msun/yr, resulting in a metal enrichment of 1e-3 solar for the continuous formation of stars. Such systems may have been more common in the past and a similar enrichment level is measured for the `metallicity floor' in damped Lyman-alpha absorption systems.

We present the HIPASS Bright Galaxy Catalog (BGC), which contains the 1000 H I brightest galaxies in the southern sky as obtained from the H I Parkes All-Sky Survey (HIPASS). The selection of the brightest sources is based on their H I peak flux density (Speak ≳ 116 mJy) as measured from the spatially integrated HIPASS spectrum. The derived H I masses range from ∼107 to 4 × 1010 M⊙. While the BGC (z < 0.03) is complete in Speak, only a subset of ∼500 sources can be considered complete in integrated H I flux density (F ≳ 25 Jy km s-1). The HIPASS BGC contains a total of 158 new redshifts. These belong to 91 new sources for which no optical or infrared counterparts have previously been cataloged, an additional 51 galaxies for which no redshifts were previously known, and 16 galaxies for which the cataloged optical velocities disagree. Of the 91 newly cataloged BGC sources, only four are definite H I clouds: while three are likely Magellanic debris with velocities around 400 km s-1, one is a tidal cloud associated with the NGC 2442 galaxy group. The remaining 87 new BGC sources, the majority of which lie in the zone of avoidance, appear to be galaxies. We identified optical counterparts to all but one of the 30 new galaxies at Galactic latitudes |b| > 10°. Therefore, the BGC yields no evidence for a population of "free-floating" intergalactic H I clouds without associated optical counterparts. HIPASS provides a clear view of the local large-scale structure. The dominant features in the sky distribution of the BGC are the Supergalactic Plane and the Local Void. In addition, one can clearly see the Centaurus Wall, which connects via the Hydra and Antlia Clusters to the Puppis Filament. Some previously hardly noticable galaxy groups stand out quite distinctly in the H I sky distribution. Several new structures, including some not behind the Milky Way, are seen for the first time.

The H I Parkes All Sky Survey (HIPASS) is a blind extragalactic H I 21-cm emission-line survey covering the whole southern sky from declination -90degrees to +25degrees. The HIPASS catalogue (HICAT), containing 4315 H I-selected galaxies from the region south of declination +2degrees, is presented in Meyer et al. (Paper I). This paper describes in detail the completeness and reliability of HICAT, which are calculated from the recovery rate of synthetic sources and follow-up observations, respectively. HICAT is found to be 99 per cent complete at a peak flux of 84 mJy and an integrated flux of 9.4 Jy km. s(-1). The overall reliability is 95 per cent, but rises to 99 per cent for sources with peak fluxes >58 mJy or integrated flux >8.2 Jy km s(-1). Expressions are derived for the uncertainties on the most important HICAT parameters: peak flux, integrated flux, velocity width and recessional velocity. The errors on HICAT parameters are dominated by the noise in the HIPASS data, rather than by the parametrization procedure.

Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the Local Group model and would have HI volume densities of order 0.0003/cm^3. Clouds with these volume densities and the observed neutral hydrogen column densities will be largely ionized, even if exposed only to the extragalactic ionizing radiation field. Here we examine the implications of this process for models of CHVCs. We have modeled the ionization structure of spherical clouds (with and without dark matter halos) for a large range of densities and sizes, appropriate to CHVCs over the range of suggested distances, exposed to the extragalactic ionizing photon flux. Constant-density cloud models in which the CHVCs are at Local Group distances have total (ionized plus neutral) gas masses roughly 20-30 times larger than the neutral gas masses, implying that the gas mass alone of the observed population of CHVCs is about 40 billion solar masses. With a realistic (10:1) dark matter to gas mass ratio, the total mass in such CHVCs is a significant fraction of the dynamical mass of the Local Group, and their line widths would exceed the observed FWHM. Models with dark matter halos fare even more poorly; they must lie within approximately 200 kpc of the Galaxy. We show that exponential neutral hydrogen column density profiles are a natural consequence of an external source of ionizing photons, and argue that these profiles cannot be used to derive model-independent distances to the CHVCs. These results argue strongly that the CHVCs are not cosmological objects, and are instead associated with the Galactic halo.

The Magellanic Stream is IGM fuel which originates from the interaction of the Large and Small Magellanic Clouds (LMC & SMC) with each other and the Galaxy. The Stream trails the Clouds for over 100° and serves as a probe of the mass and density of the Galactic halo. Figure 1 shows the column density distribution of the Magellanic System (N HI > 2 x 1018 cm-2) using HVC reduced HIPASS data (-500 > V LSR < 500 km s-1; excluding ± 90 km s-1; see Putman et al. 2002a). The Stream extends over 700 km s-1, or 400 km s-1 in terms of a Galactic reference frame, from l,b ∼ 290°, -45° to l, b ∼ 90°, -35°. Small clumps of Magellanic debris follow the length of the Stream in position and velocity. The Leading Arm (the HI clouds on the opposite side of the Magellanic Clouds to the Magellanic Stream) shows that the interaction between the Clouds and the Galaxy is predominantly a tidal one; however, these tidal features are most likely being shaped by a Galactic halo medium. The entire system contains 1.2 x 109 M⊙ of neutral hydrogen (at an average distance of 55 kpc), or ∼l/3 the Hi mass of the Galaxy.

We present deep Hα spectroscopy toward several high-velocity clouds (HVCs), which vary in structure from compact HVCs (CHVCs) to the Magellanic Stream. The clouds range from being bright (~640 mR) to having upper limits on the order of 30-70 mR. The Hα measurements are discussed in relation to their H I properties, and distance constraints are given to each of the complexes based on esc ≈ 6% of the ionizing photons escaping normal to the Galactic disk (fesc ≈ 1%-2% when averaged over solid angle). The results suggest that many HVCs and CHVCs are within a ~40 kpc radius from the Galaxy and are not members of the Local Group at megaparsec distances. However, the Magellanic Stream is inconsistent with this model and needs to be explained. It has bright Hα emission and little [N II] emission and appears to fall into a different category than the currently detected HVCs. This may reflect the lower metallicities of the Magellanic Clouds compared to the Galaxy, but the strength of the Hα emission cannot be explained solely by photoionization from the Galaxy. The interaction of the Magellanic Stream with halo gas or the presence of yet unassociated young stars may assist in ionizing the Magellanic Stream.

We present a new, accurate measurement of the H I mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest H I peak flux densities in the southern (delta<0&DEG;) hemisphere. This sample spans nearly 4 orders of magnitude in H I mass [ log (M-H I/M-&ODOT;) + 2 log h(75)=6.8-10.6] and is the largest sample of H I-selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies and show that this is a robust method, insensitive to the effects of large-scale structure. The resulting H I mass function can be fitted satisfactorily with a Schechter function with faint-end slope &alpha;=-1.30. This slope is found to be dependent on morphological type, with late-type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the H I mass function, including peculiar motions of galaxies, large-scale structure, selection bias, and inclination effects, and we quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: &UOmega;(H) I=(3.8&PLUSMN;0.6)x10(-4) h(75)(-1). Low surface brightness galaxies contribute only similar to15% to this value, consistent with previous findings.

The Magellanic Stream is a 100° × 10° filament of gas that lies within the Galactic halo and contains ~2 × 108 M☉ of neutral hydrogen. In this paper we present data from the H I Parkes All Sky Survey (HIPASS) in the first complete survey of the entire Magellanic Stream and its surroundings. We also present a summary of the reprocessing techniques used to recover large-scale structure in the Stream. The substantial improvement in spatial resolution and angular coverage compared to previous surveys reveals a variety of prominent features, including bifurcation along the main Stream filament; dense, isolated clouds that follow the entire length of the Stream; head-tail structures; and a complex filamentary web at the head of the Stream where gas is being freshly stripped away from the Small Magellanic Cloud and the Bridge. Debris that appears to be of Magellanic origin extends out to 20° from the main Stream filaments. The Magellanic Stream has a velocity gradient of 700 km s-1 from the Clouds to the tail of the Stream, ~390 km s-1 greater than that due to Galactic rotation alone, therefore implying a noncircular orbit. The dual filaments comprising the Stream are likely to be relics from gas stripped separately from the Magellanic Bridge and the SMC. This implies that (1) the Bridge is somewhat older than conventionally assumed; and (2) the Clouds have been bound together for at least one or two orbits. The transverse velocity gradient of the Stream also appears to support long-term binary motion of the Clouds. A significant number of the most elongated cataloged Stream clouds (containing ~1% of the Stream mass) have position angles aligned along the Stream. This suggests the presence of shearing motions within the Stream, arising from tidal forces or interaction with the tenuous Galactic halo. As previously noted, clouds within one region of the Stream, along the sight line to the less distant half (southern half on the sky) of the Sculptor Group, show anomalous properties. There are more clouds along this sight line than any other part of the Stream, and their velocity distribution significantly deviates from the gradient along the Stream. We argue that this deviation could be due to a combination of halo material, and not to distant Sculptor clouds, based on a spatial and kinematic comparison between the Sculptor Group galaxies and the anomalous clouds and the lack of cloud detection in the northern half of the group. This result has significant implications for the hypothesis that there might exist distant, massive high-velocity clouds within the Local Group. Cataloged clouds within the Magellanic Stream do not have a preferred scale size. Their mass spectrum f(M) ∝ M and column density spectrum f(N) ∝ N are steep compared with Lyα absorbers and galaxies, and similar to the anomalous clouds along the Sculptor Group sight line.

We report on H I observations with the Australia Telescope and the Parkes telescope of one of the cores of the high-velocity cloud (HVC) WW 187 (also known as HVC 287+23+240 and HVC 287.5+22.5+240). With our H I map, this becomes the first HVC for which single-dish and interferometer data have been combined. The analysis of the H I fine structure shows that the smallest concentrations are almost resolved and may be as small as 5-15 pc. The structure of the cloud is hierarchical: smaller, denser cores are embedded in a smoother envelope. At 1' resolution, the peak brightness temperatures of the cores lie in the range 15-35 K, while the FWHMs of the individual spectra range from 3 to 9 km s(-1), with a modal value of similar to5 km s(-1). This limits the kinetic temperature of the gas to lie between 35 and 500 K. The peak column densities range from 1.0 x 10(20) to 3.6 x 10(20) cm(-2). The distribution of column densities in the field is an exponential: count proportional to exp [-N(H I)]. The volume densities in the cores could be as high as 20 cm(-3), which is sufficiently high to provide a long-time source for the molecular hydrogen in the HVC that is seen in the direction of NGC 3783. The original motivation for this observation came from the detection of interstellar UV absorption lines in the spectrum of the extragalactic background source NGC 3783 from which metal abundances for WW 187 were derived that are similar to those in the SMC. We describe in detail the derivation of the value for N(H I) used to derive these abundances. In combination with model calculations, these abundances support the contention that WW 187 is part of the leading arm of the Magellanic Stream and that the Magellanic Stream is a tidal feature, rather than being formed by ram pressure.

A homogeneous set of ground-based BVRI observations of similar to600 Cepheids is presented to check the Cepheid period-luminosity zero point for the Large Magellanic Cloud (LMC). The sample of Cepheids is completely self-contained and has been reduced consistently to eliminate photometric differences caused by combining multiple sources of photometry.The Cepheid distances to nearby galaxies obtained by the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale were computed assuming a "standard" PL relation based on a compilation of photoelectric observations of 34 Cepheids and a standard distance modulus to the LMC of 18.50. The final Key Project results substituted a statistically stronger PL relation obtained by the OGLE collaboration. In this paper we compare our data with the OGLE PL relation; the subsample of LMC Cepheids with similar periods to those discovered in the nearby galaxies is fainter at the relevant periods by 0.04 +/- 0.02 mag than the OGLE sample. Substituting in turn this PL relation for that of the OGLE collaboration would raise the Hubble constant by 2% +/- 1%, a correction which is not significant.The multicolor data set will also be useful in considering the effects of reddening and chemical composition and deriving an improved Cepheid period-luminosity relation for the LMC.

We present a comparison of the leftover satellites at z=0 in a cold dark matter dominated simulation of the formation of the Local Group to the distribution of observed neutral hydrogen high-velocity clouds and compact high-velocity clouds. The ~2000 leftover satellites in the simulation have dark matter masses which range between 0.5 to 10 x 10^9 Msun, sizes between 3 to 30 kpc, and distances between 100 kpc and 2 Mpc. The dark matter halos show a clear bias in their distribution towards M31 and to a lesser extent towards the Local Group anti-barycenter. If the Local Group halos contain ~1% of their dark matter mass in neutral hydrogen they should have been easily detected by the current HI surveys. The only HI objects detected with the potential to be the Local Group halos are the high-velocity clouds. Here the spatial, kinematic, and HI flux properties of the clouds and dark matter halos are compared. Several different subsets of halos which may be more likely to contain neutral hydrogen are investigated, and the HVCs are found to have some similar properties to those halos within 500 kpc of the Galaxy and those halos with dark matter masses > 2 x 10^8 Msun. The compact high-velocity clouds do not show similar properties to the halos.

There is a fortuitous coincidence in the positions of the quasar Ton S210 and the compact H I high-velocity cloud CHVC 224.0-83.4-197 on the sky. Using Far Ultraviolet Spectroscopic Explorer observations of the metal line absorption in this cloud and sensitive H I 21 cm emission observations obtained with the multi-beam system at Parkes Observatory, we determine a metallicity of (O/H) < 0.46 solar at a confidence of 3 sigma. The metallicity of the high-velocity gas is consistent with either an extragalactic or Magellanic Cloud origin but is not consistent with a location inside the Milky Way unless the chemical history of the gas is considerably different from that of the interstellar medium in the Galactic disk and halo. Combined with measurements of highly ionized species (C III and O vI) at high velocities, this metallicity limit indicates that the cloud has a substantial halo of ionized gas; there is as much ionized gas as neutral gas directly along the Ton S210 sight line. We suggest several observational tests that would improve the metallicity determination substantially and help to distinguish between possible origins for the high-velocity gas. Additional observations of this sight line would be valuable, since the number of compact HVCs positioned in front of background sources bright enough for high-resolution absorption-line studies is extremely limited.

Using data from the H I Parkes All Sky Survey (HIPASS), we have searched for neutral hydrogen in galaxies in a region similar to25x25 deg(2) centred on NGC 1399, the nominal centre of the Fornax cluster. Within a velocity search range of 300-3700 km s(-1) and to a 3sigma lower flux limit of similar to40 mJy, 110 galaxies with H I emission were detected, one of which is previously uncatalogued. None of the detections has early-type morphology. Previously unknown velocities for 14 galaxies have been determined, with a further four velocity measurements being significantly dissimilar to published values. Identification of an optical counterpart is relatively unambiguous for more than similar to90 per cent of our H I galaxies. The galaxies appear to be embedded in a sheet at the cluster velocity which extends for more than 30degrees across the search area. At the nominal cluster distance of similar to20 Mpc, this corresponds to an elongated structure more than 10 Mpc in extent. A velocity gradient across the structure is detected, with radial velocities increasing by similar to500 km s(-1) from south-east to north-west. The clustering of galaxies evident in optical surveys is only weakly suggested in the spatial distribution of our H I detections. Of 62 H I detections within a 10degrees projected radius of the cluster centre, only two are within the core region (projected radius <1&DEG;) and less than 30 per cent are within 3.5&DEG;, suggesting a considerable deficit of H I-rich galaxies in the centre of the cluster. However, relative to the field, there is a 3(&PLUSMN;1)-fold excess of H I-rich galaxies in the outer parts of the cluster where galaxies may be infalling towards the cluster for the first time.