A multibeam HI survey of the Virgo cluster - two isolated HI clouds?

We have carried out a fully sampled large area (4degrees x 8degrees) 21-cm H I line survey of part of the Virgo cluster using the Jodrell Bank multibeam instrument. The survey has a sensitivity some three times better than the standard HIJASS (H I Jodrell All Sky Survey) and HIPASS (H I Parkes All Sky Survey) surveys. We detect 31 galaxies, 27 of which are well-known cluster members. The four new detections have been confirmed in the HIPASS data and by follow-up Jodrell Bank pointed observations. One object lies behind M86, but the other three have no obvious optical counterparts upon inspection of the digital sky survey fields. These three objects were mapped at Arecibo with a smaller 3.6-arcmin half power beam width (HPBW) and a four times better sensitivity than the Jodrell Bank data, which allow an improved determination of the dimensions and location of two of the objects, but surprisingly failed to detect the third. The two objects are resolved by the Arecibo beam, giving them a size far larger than any optical images in the nearby field. To our mass limit of 5 x 10(7) (Deltaupsilon/50 km s(-1)) M. and column density limit of 3 x 10(18)(Deltaupsilon/50 km s(-1)) atom cm(-2), these new detections represent only about 2 per cent of the cluster atomic hydrogen mass. Our observations indicate that the H I mass function of the cluster turns down at the low-mass end, making it very different to the field galaxy H I mass function. This is quite different to the Virgo cluster optical luminosity function, which is much steeper than that in the general field. Many of the sample galaxies are relatively gas-poor compared with H I selected samples of field galaxies, confirming the I anaemic spirals' view of Virgo cluster late-type galaxies. The velocity distribution of the H I detected galaxies is also very different to that of the cluster as a whole. There are relatively more high-velocity galaxies in the H I sample, suggesting that they form part of a currently infalling population. The H I sample with optical identifications has a minimum H I column density cut-off more than an order of magnitude above that expected from the sensitivity of the survey. This observed column density is above the normally expected level for star formation to occur. The two detections with no optical counterparts have very much lower column densities than that of the rest of the sample, below the star formation threshold.