The large-scale distribution of neutral hydrogen in the Fornax region

Abstract

Using data from the HI Parkes All Sky Survey (HIPASS), we have searched for neutral hydrogen in galaxies in a region ∼25 × 25 deg2 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 3σ lower flux limit of ∼40 mJy, 110 galaxies with Hi 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 ∼90 per cent of our HI 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 ∼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 ∼500 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 HI detections. Of 62 HI 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 HI-rich galaxies in the centre of the cluster. However, relative to the field, there is a 3(±1)-fold excess of HI-rich galaxies in the outer parts of the cluster where galaxies may be infalling towards the cluster for the first time.