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

Jonathan Davies (1) , Robert F. Minchin (1) , Sabina Sabatini (1) , Willem van Driel (2, 3) , Maarten Baes (1) , Peter Boyce (1) , W. J. G. de Blok (1) , Michael J. Disney (1) , Rhodri Evans (1) , Virginia Kilborn (4) , R. Lang (1) , Suzanne Linder (1) , Sarah Roberts (1) , R. Smith (1)

1 Cardiff University
2 GEPI - Galaxies, Etoiles, Physique, Instrumentation
3 Physique des Galaxies et Cosmologie
4 Jodrell Bank Observatory, University of Manchester, Macclesfield

We have carried out a fully sampled large area (4°× 8°) 21-cm HI 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 (HI Jodrell All Sky Survey) and HIPASS (HI 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 × 10⁷(Deltav/50 km s^-1) M_solar and column density limit of 3 × 10¹⁸(Deltav/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 HI mass function of the cluster turns down at the low-mass end, making it very different to the field galaxy HI 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 HI selected samples of field galaxies, confirming the `anaemic spirals' view of Virgo cluster late-type galaxies. The velocity distribution of the HI detected galaxies is also very different to that of the cluster as a whole. There are relatively more high-velocity galaxies in the HI sample, suggesting that they form part of a currently infalling population. The HI sample with optical identifications has a minimum HI 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.

Domaines: Astrophysique [astro-ph]

Jonathan Davies, Robert F. Minchin, Sabina Sabatini, Willem van Driel, Maarten Baes, et al.. A multibeam HI survey of the Virgo cluster - two isolated HI clouds?. Monthly Notices of the Royal Astronomical Society, 2004, 349, pp.922-932. ⟨10.1111/j.1365-2966.2004.07568.x⟩. ⟨hal-03785985⟩