Search

We have placed limits on the cosmological significance of gas-rich low surface-brightness (LSB) galaxies as a proportion of the total population of gas-rich galaxies by carrying out a very deep survey (HIDEEP) for neutral hydrogen (HI) with the Parkes multibeam system. Such a survey avoids the surface-brightness selection effects that limit the usefulness of optical surveys for finding LSB galaxies. To complement the HIDEEP survey we have digitally stacked eight 1-hour R-band Tech Pan films from the UK Schmidt Telescope covering 36 square degrees of the survey area to reach a very deep isophotal limit of 26.5 R mag/sq. arcsec. At this level, we find that all of the 129 HI sources within this area have optical counterparts and that 107 of them can be identified with individual galaxies. We have used the properties of the galaxies identified as the optical counterparts of the HI sources to estimate the significance of LSB galaxies (defined to be those at least 1.5 magnitudes dimmer in effective surface-brightness than the peak in the observed distribution seen in optical surveys). We calculate the contribution of LSB galaxies to the total number, neutral hydrogen density, luminosity density, baryonic mass density, dynamical mass density and cross-sectional area of gas-rich galaxies. We do not find any `Crouching Giant' LSB galaxies such as Malin 1, nor do we find a population of extremely low surface-brightness galaxies not previously found by optical surveys. Such objects must either be rare, gas-poor or outside the survey detection limits., Comment: 13 pages, 14 figures. Accepted for publication in MNRAS

We have carried out an extremely long integration-time (9000 s/beam) 21-cm blind survey of 60 square degrees in Centaurus using the Parkes multibeam system. We find that the noise continues to fall as sqrt(t) throughout, enabling us to reach an HI column-density limit of 4.2 x 10^18 cm^-2 for galaxies with a velocity width of 200 km/s in the central 32 square degree region, making this the deepest survey to date in terms of column density sensitivity. The HI data are complemented by very deep optical observations from digital stacking of multi-exposure UK Schmidt Telescope R-band films, which reach an isophotal level of 26.5 R mag arcsec^-2 (~27.5 B mag arcsec^-2). 173 HI sources have been found, 96 of which have been uniquely identified with optical counterparts in the overlap area. There is not a single source without an optical counterpart. Although we have not measured the column-densities directly, we have inferred them from the optical sizes of their counterparts. All appear to have a column-density of N_HI = 10^(20.65 +/- 0.38). This is at least an order of magnitude above our sensitivity limit, with a scatter only marginally larger than the errors on N_HI. This needs explaining. If confirmed it means that HI surveys will only find low surface brightness (LSB) galaxies with high M_HI/L_B. Gas-rich LSB galaxies with lower HI mass to light ratios do not exist. The paucity of low column-density galaxies also implies that no significant population will be missed by the all-sky HI surveys being carried out at Parkes and Jodrell Bank., Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society. 18 pages, 23 figures

We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of a gas cloud associated with the asymmetric spiral galaxy NGC 2442. This object, designated HIPASS J0731-69, contains ~10^9 M_sun of HI, or nearly one-third as much atomic gas as NGC 2442 itself. No optical counterpart to any part of HIPASS J0731-69 has yet been identified, consistent with the gas being diffuse, and with its stream-like kinematics. If the gas in HIPASS J0731-69 was once part of NGC 2442, then it was most likely a fairly recent tidal encounter with a moderately massive companion which tore it loose, although the possibility of ram-pressure stripping cannot be ruled out. This discovery highlights the potential of the HIPASS data for yielding new clues to the nature of some of the best-known galaxies in the local universe., Comment: 8 pages, 4 figures, uses "emulateapj5.sty". Accepted for publication in ApJ, Vol. 555, 1 July 2001. Figs 1 and 2 included as JPEG

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<2000 K) rather than bulk motion or turbulence. The peak column density for HIPASS J1712-64, from the combined Parkes and ATCA data, is only 3.5 x 10^19 cm^-2, which is estimated to be a factor of two below the critical threshold for star formation. Apart from its significantly higher velocity, the properties of HIPASS J1712-64 are similar to the recently recognised class of Compact High Velocity Clouds. We therefore consider the evidence for a Local Group or Galactic origin, although a more plausible alternative is that HIPASS J1712-64 was ejected from the interacting Magellanic Cloud/Galaxy system at perigalacticon ~ 2 x 10^8 yr ago., Comment: 23 pages, 7 figures, AJ accepted

We have commenced a 21-cm survey of the entire southern sky (\delta < 0 degrees, -1200 km/s < v < 12700 km/s) which is ''blind'', i.e. unbiased by previous optical information. In the present paper we report on the results of a pilot project which is based on data from this all-sky survey. The project was carried out on an area of 600 square degrees centred on the nearby Centaurus A (Cen A) group of galaxies at a mean velocity of v ~ 500 km/s. This was recently the subject of a separate and thorough optical survey. We found 10 new group members to add to the 21 galaxies already known in the Cen A group: five of these are previously uncatalogued galaxies, while five were previously catalogued but not known to be associated with the group. We found optical counterparts for all the HI detections, most of them intrinsically very faint low surface brightness dwarfs. The new group members add approximately 6% to the HI mass of the group and 4% to its light. The HI mass function, derived from all the known group galaxies in the interval 10^7 \Msun of HI to 10^9 \Msun of HI, has a faint-end slope of 1.30 +/- 0.15, allowing us to rule out a slope of 1.7 at 95% confidence. Even if the number in the lowest mass bin is increased by 50%, the slope only increases to 1.45 +/- 0.15., Comment: 19 pages Latex, 6 figures (Fig. 2 in four parts, Fig.5 in two parts). To appear in The Astrophysical Journal (Vol. 524, October 1999)

Interactions between galaxies are common and are an important factor in determining their physical properties such as position along the Hubble sequence and star-formation rate. There are many possible galaxy interaction mechanisms, including merging, ram-pressure stripping, gas compression, gravitational interaction and cluster tides. The relative importance of these mechanisms is often not clear, as their strength depends on poorly known parameters such as the density, extent and nature of the massive dark halos that surround galaxies. A nearby example of a galaxy interaction where the mechanism is controversial is that between our own Galaxy and two of its neighbours -- the Large and Small Magellanic Clouds. Here we present the first results of a new HI survey which provides a spectacular view of this interaction. In addition to the previously known Magellanic Stream, which trails 100 degrees behind the Clouds, the new data reveal a counter-stream which lies in the opposite direction and leads the motion of the Clouds. This result supports the gravitational model in which leading and trailing streams are tidally torn from the body of the Magellanic Clouds., Comment: 17 pages with 5 figures in gif format, scheduled for publication in the August 20th, 1998 issue of Nature