VIRGOHI21 is an HI source detected in the Virgo Cluster survey of Davies et al. (2004) which has a neutral hydrogen mass of 10^8 M_solar and a velocity width of Delta V_20 = 220 km/s. From the Tully-Fisher relation, a galaxy with this velocity width would be expected to be 12th magnitude or brighter; however deep CCD imaging has failed to turn up a counterpart down to a surface-brightness level of 27.5 B mag/sq. arcsec. The HI observations show that it is extended over at least 16 kpc which, if the system is bound, gives it a minimum dynamical mass of ~10^11 M_solar and a mass to light ratio of M_dyn/L_B > 500 M_solar/L_solar. If it is tidal debris then the putative parents have vanished; the remaining viable explanation is that VIRGOHI21 is a dark halo that does not contain the expected bright galaxy. This object was found because of the low column density limit of our survey, a limit much lower than that achieved by all-sky surveys such as HIPASS. Further such sensitive surveys might turn up a significant number of the dark matter halos predicted by Dark Matter models.Comment: Accepted by ApJ

Results are presented of the first blind HI survey of the M81 group of galaxies. The data were taken as part of the HI Jodrell All Sky Survey (HIJASS). The survey reveals several new aspects to the complex morphology of the HI distribution in the group. All four of the known dwarf irregular (dIrr) galaxies close to M81 can be unambiguously seen in the HIJASS data. Each forms part of the complex tidal structure in the area. We suggest that at least three of these galaxies may have formed recently from the tidal debris in which they are embedded. The structure connecting M81 to NGC2976 is revealed as a single tidal bridge of mass ≃2.1×10 8 M ⊙ and projected spatial extent ≃80 kpc. Two 'spurs' of HI projecting from the M81 complex to lower declinations are traced over a considerably larger spatial and velocity extent than by previous surveys. The dwarf elliptical (dE) galaxies BK5N and Kar 64 lie at the spatial extremity of one of these features and appear to be associated with it. We suggest that these may be the remnants of dIrrs which have been stripped of gas and transmuted into dEs by close gravitational encounters with NGC3077. The nucleated dE galaxy Kar 61 is unambiguously detected in HI for the first time and has an HI mass of ∼10 8 M ⊙ , further confirming it as a dE/dIrr transitional object. HIJASS has revealed one new possible group member, HIJASS J1021+6842. This object contains ≃2×10 7 M ⊙ of HI and lies ≃105 ′ from IC2574. It has no optical counterpart on the Digital Sky Survey.

Details are presented of the H i Jodrell All Sky Survey (HIJASS). HIJASS is a blind neutral hydrogen (H i) survey of the northern sky (δ > 22°), being conducted using the multibeam receiver on the Lovell Telescope (full width at half‐maximum beamwidth 12 arcmin) at Jodrell Bank. HIJASS covers the velocity range −3500 to 10 000 km s−1, with a velocity resolution of 18.1 km s−1 and spatial positional accuracy of ∼2.5 arcmin. Thus far about 1115 deg2 of sky have been surveyed. The average rms noise during the early part of the survey was around 16 mJy beam−1. Following the first phase of the Lovell Telescope upgrade (in 2001), the rms noise is now around 13 mJy beam−1. We describe the methods of detecting galaxies within the HIJASS data and of measuring their H i parameters. The properties of the resulting H i‐selected sample of galaxies are described. Of the 222 sources so far confirmed, 170 (77 per cent) are clearly associated with a previously catalogued galaxy. A further 23 sources (10 per cent) lie close (within 6 arcmin) to a previously catalogued galaxy for which no previous redshift exists. A further 29 sources (13 per cent) do not appear to be associated with any previously catalogued galaxy. The distributions of peak flux, integrated flux, H i mass and cz are discussed. We show, using the HIJASS data, that H i self‐absorption is a significant, but often overlooked, effect in galaxies with large inclination angles to the line of sight. Properly accounting for it could increase the derived H i mass density of the local Universe by at least 25 per cent. The effect that this will have on the shape of the H i mass function will depend on how self‐absorption affects galaxies of different morphological types and H i masses. We also show that galaxies with small inclinations to the line of sight may also be excluded from H i‐selected samples, since many such galaxies will have observed velocity widths that are too narrow for them to be distinguished from narrow‐band radio‐frequency interference. This effect will become progressively more serious for galaxies with smaller intrinsic velocity widths. If, as we might expect, galaxies with smaller intrinsic velocity widths have smaller H i masses, then compensating for this effect could significantly steepen the faint‐end slope of the derived H i mass function.

Many observations indicate that dark matter dominates the extragalactic universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously, we have suggested that VIRGOHI 21, a 21 cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line width ($200 km s À1 ) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction $10 8 yr ago with a perturber of $10 11 M . This we take as additional evidence for the massive nature of VIRGOHI 21, as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the H i and find none down to an I-band surface brightness limit of 31:1 AE 0:2 mag arcsec À2 .

We have conducted the first blind H I survey covering 480 deg 2 and a heliocentric velocity range from 300-1900 km s −1 to investigate the H I content of the nearby spiral-rich Ursa Major region and to look for previously uncatalogued gas-rich objects. Here we present the catalog of H I sources. The H I data were obtained with the 4-beam receiver mounted on the 76.2-m Lovell telescope (FWHM 12 arcmin) at the Jodrell Bank Observatory (UK) as part of the H I Jodrell All Sky Survey (HIJASS). We use the automated source finder DUCHAMP and identify 166 H I sources in the data cubes with H I masses in the range of 10 7 -10 10.5 M . Our Ursa Major H I catalogue includes 10 first time detections in the 21-cm emission line.We identify optical counterparts for 165 H I sources (99 per cent). For 54 H I sources (∼ 33 per cent) we find numerous optical counterparts in the HIJASS beam, indicating a high density of galaxies and likely tidal interactions. Four of these H I systems are discussed in detail.We find only one H I source (1 per cent) without a visible optical counterpart out of the 166 H I detections. Green Bank Telescope (FWHM 9 arcmin) follow-up observations confirmed this H I source and its H I properties. The nature of this detection is discussed and compared to similar sources in other H I surveys.