Using data from the H i Parkes All Sky Survey ( HIPASS ) , we have searched for neutral hydrogen in galaxies in a region \sim 25 \times 25 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 3 \sigma lower flux limit of \sim 40 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 4 velocity measurements being significantly dissimilar to published values .
Identification of an optical counterpart is relatively unambiguous for more than \sim 90 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 30° across the search area .
At the nominal cluster distance of \sim 20 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 \sim 500 km s ^ { -1 } from SE to NW .
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 10° projected radius of the cluster centre , only two are within the core region ( projected radius < 1° ) and less than 30 per cent are within 3.5° , suggesting a considerable deficit of H i -rich galaxies in the centre of the cluster .
However , relative to the field , there is a 3 ( \pm 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 .