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Dark Matter supposedly dominates the extragalactic, yet no totally dark structure of galactic proportions has ever been convincingly identified. Earlier (Minchin et al. 2005) we suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster at Jodrell Bank using single-dish observations (Davies et al. 2004), was probably such a dark galaxy because of its broad line-width (~ 200 km/s) unaccompanied by any visible gravitational source to account for it. Now we have managed to image VIRGOHI 21 in the neutral-hydrogen line, and indeed we find what appears to be a dark, edge-on, spinning disc with the mass and diameter of a typical spiral galaxy. Moreover the disc has unquestionably interacted with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor invariably responsible for such a feature. Published numerical models (Vollmer, Huchtmeier & van Driel 2005) of NGC 4254 call for a close interaction ~ 10^8 years ago with a perturber of 10^11 solar masses. This we take as completely independent evidence for the massive nature of VIRGOHI 21., 5 pages, submitted to MNRAS. Animation available at http://www.astro.cf.ac.uk/groups/galaxies

In a recent paper Lehnert & Bremer have photometrically selected a sample of galaxies at z>4.8 from a single VLT/FORS2 pointing and spectroscopically confirmed half of them to be at 4.828, i(AB)0) from the HST ACS images of the Chandra Deep Field South. This selection results in a sample of 44 sources from ~150 sq. arcmin. We find that such galaxies are often barely resolved in the ACS images, having half-light radii of 0.1-0.3 arcsec (0, which are generally galaxies of lower redshift. However, their distribution over the field is not uniform and their surface density varies considerably over areas comparable to a single 8m or HST pointing. The reliable determination of the surface and volume densities of such galaxies requires a sky area considerably larger than the current ACS imaging of this field. No individual z>5 candidate was detected to a 3-sigma limit of 6 x 10^-17 erg s^-1 cm^-2 at 0.5-5 keV by Chandra (a limiting luminosity of below 2 x 10^43 erg s^-1 at z~5.3). By summing over all positions, we find that the mean source must be undetected at a level at least a factor 4 times fainter than this. This rules out anything other than a weak AGN contribution to the emission from these objects and thus luminous AGN made little contribution to the final stages of re-ionization of the Universe.

We present a new accurate measurement of the HI mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest HI peak flux densities in the southern hemisphere (Koribalski et al. 2003). This sample spans nearly four orders of magnitude in HI mass (from log M_HI/M_sun=6.8 to 10.6, H0=75) and is the largest sample of HI selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies, and show that this is a robust method, insensitive to the effects of large scale structure. The resulting HI mass function can be fitted satisfactorily with a Schechter function with faint-end slope alpha=-1.30. This slope is found to be dependent on morphological type, with later type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the HI mass function, including peculiar motions of galaxies, large scale structure, selection bias, and inclination effects, and quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: Omega_HI=(3.8 +/- 0.6) x 10^{-4}. Low surface brightness galaxies contribute only 15% to this value, consistent with previous findings., Comment: accepted for publication in Astronomical Journal, 16 pages, including 17 figures. Corrected typos and references

In a recent paper Lehnert & Bremer have photometrically selected a sample of galaxies at z>4.8 from a single VLT/FORS2 pointing and spectroscopically confirmed half of them to be at 4.828, i(AB)0) from the HST ACS images of the Chandra Deep Field South. This selection results in a sample of 44 sources from ~150 sq. arcmin. We find that such galaxies are often barely resolved in the ACS images, having half-light radii of 0.1-0.3 arcsec (0, which are generally galaxies of lower redshift. However, their distribution over the field is not uniform and their surface density varies considerably over areas comparable to a single 8m or HST pointing. The reliable determination of the surface and volume densities of such galaxies requires a sky area considerably larger than the current ACS imaging of this field. No individual z>5 candidate was detected to a 3-sigma limit of 6 x 10^-17 erg s^-1 cm^-2 at 0.5-5 keV by Chandra (a limiting luminosity of below 2 x 10^43 erg s^-1 at z~5.3). By summing over all positions, we find that the mean source must be undetected at a level at least a factor 4 times fainter than this. This rules out anything other than a weak AGN contribution to the emission from these objects and thus luminous AGN made little contribution to the final stages of re-ionization of the Universe.

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 approx. 2.1 x 10^8 Msol and projected spatial extent approx. 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 has 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 approx.10^8 Msol, further confirming it as a dE/dIrr transitional object. HIJASS has revealed one new possible group member, HIJASS J1021+6842. This object contains approx. 2 x 10^7 Msol of HI and lies approx.105arcmin from IC2574. It has no optical counterpart on the Digital Sky Survey., To be published in Astrophysical Journal Letters 9 pages, including 3 figures

We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of an isolated cloud of neutral hydrogen which we believe to be extragalactic. The HI mass of the cloud (HIPASS J1712-64) is very low, 1.7 x 10^7 Msun, using an estimated distance of ~3.2 Mpc. Most significantly, we have found no optical companion to this object to very faint limits (mu(B)~ 27 mag arcsec^-2). HIPASS J1712-64 appears to be a binary system similar to, but much less massive than, HI 1225+01 (the Virgo HI Cloud) and has a size of at least 15 kpc. The mean velocity dispersion, measured with the Australia Telescope Compact Array (ATCA), is only 4 km/s for the main component and because of the weak or non-existent star-formation, possibly reflects the thermal linewidth (T, 23 pages, 7 figures, AJ accepted