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We present a comparison of the leftover satellites at z=0 in a cold dark matter dominated simulation of the formation of the Local Group to the distribution of observed neutral hydrogen high-velocity clouds and compact high-velocity clouds. The ~2000 leftover satellites in the simulation have dark matter masses which range between 0.5 to 10 x 10^9 Msun, sizes between 3 to 30 kpc, and distances between 100 kpc and 2 Mpc. The dark matter halos show a clear bias in their distribution towards M31 and to a lesser extent towards the Local Group anti-barycenter. If the Local Group halos contain ~1% of their dark matter mass in neutral hydrogen they should have been easily detected by the current HI surveys. The only HI objects detected with the potential to be the Local Group halos are the high-velocity clouds. Here the spatial, kinematic, and HI flux properties of the clouds and dark matter halos are compared. Several different subsets of halos which may be more likely to contain neutral hydrogen are investigated, and the HVCs are found to have some similar properties to those halos within 500 kpc of the Galaxy and those halos with dark matter masses > 2 x 10^8 Msun. The compact high-velocity clouds do not show similar properties to the halos., Comment: 11 pages (incl. 5 figures), to appear in "Extragalactic Gas at Low Redshift", ed. J. Mulchaey & J. Stocke, ASP conference series, in press 2002

Significant numbers of high-velocity HI clouds (HVCs) have now been detected in H-alpha, with a subset seen in low ionization lines (e.g. [NII]). It was originally hoped that the observed H-alpha strength would provide a distance constraint to individual clouds. This idea requires that a useful fraction (f_esc > 1%) of ionizing photons escape the Galaxy, and that the halo ionizing field is relatively smooth, as we discuss. Most HVCs which are known to be close to the Sun are H-alpha; the brightest clouds also show enhanced [NII] emission, in contrast to the Magellanic Stream where the low ionization emission lines are always weak compared to H-alpha. But an acute complication for H-alpha distances is the apparent H-alpha brightness of the Magellanic Stream along several sight lines. To account for this, we present three possible configurations for the Magellanic Stream and propose a follow-up experiment. If we normalize the distances to local HVCs, some HVCs appear to be scattered throughout the Galactic halo on scales of tens of kiloparsecs., Comment: 14 pages, 6 figures; delayed submission due to embargo on HIPASS map. To appear in Gas & Galaxy Evolution, ASP Conf. 240, 369-382 (2001), eds. J.E. Hibbard, M.P. Rupen, J.H. van Gorkom

We show 21-cm line profiles in the direction of stars and extragalactic objects, lying projected on high- and intermediate-velocity clouds (HVCs and IVCs). About half of these are from new data obtained with the Effelsberg 100-m telescope, about a quarter are extracted from the Leiden-Dwingeloo Survey (LDS) and the remaining quarter were observed with other single-dish telescopes. HI column densities were determined for each HVC/IVC. Wakker (2001) (Paper I) uses these in combination with optical and ultraviolet high-resolution measurements to derive abundances. Here, an analysis is given of the difference and ratio of N(HI) as observed with a 9 arcmin versus a 35 arcmin beam. For HVCs and IVCs the ratio N(HI-9 arcmin)/N(HI-35 arcmin) lies in the range 0.2-2.5. For low-velocity gas this ratio ranges from 0.75 to 1.3 (the observed ratio is 0.85-1.4, but it appears that the correction for stray radiation is slightly off). The smaller range for the low-velocity gas may be caused by confusion in the line of sight, so that a low ratio in one component can be compensated by a high ratio in another -- for 11 low-velocity clouds fit by one component the distribution of ratios has a larger dispersion. Comparison with higher angular resolution data is possible for sixteen sightlines. Eight sightlines with HI data at 1 arcmin-2 arcmin resolution show a range of 0.75-1.25 for N(HI-2 arcmin)/N(HI-9 arcmin), while in eight other sightlines N(HI-Ly-alpha)/N(HI-9 arcmin) ranges from 0.74 to 0.98., Comment: To appear in the "Astrophysical Journal Supplement"; 45 pages; degraded figures (astro-ph restriction) - ask for good versions

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

The results of the interaction between the Milky Way and the Magellanic Clouds are revealed through several high velocity complexes which are connected to the Clouds. The exact mechanism of their formation is under some debate, but they remain the only group of high-velocity clouds (HVCs) for which we have an origin and roughly a distance. Given that, the Magellanic HVCs can be used as a calibrator for other HVCs, while also providing an opportunity to closely investigate the remnants of an interacting system. These HVCs may hold the key to the star formation history, kinematic structure, and present Hubble type of the Magellanic Clouds, and their proximity to the Milky Way allows us to estimate key Galactic parameters. The HVCs related to the Magellanic System can be classified into three major complexes: the Magellanic Bridge, an HI connection between the Clouds; the Magellanic Stream, which trails the clouds and is one of the largest HI features in the sky outside of our Galaxy; and the Leading Arm, a more diffuse HI filament which leads the Clouds. In terms of HVCs, these features have been studied rather extensively. In this review, I will first describe the observational results for each complex and subsequently discuss their origin and relationship to the overall HVC population. All of the HI data presented are from the HI Parkes All-Sky Survey (HIPASS) (Barnes et al. 2001; Putman et al. 2001)., Comment: 21 pages, 6 figures in gif format, chapter for the book "High-Velocity Clouds" (Kluwer: Dordrecht), edited by H. van Woerden, U. Schwarz, B. Wakker and K. de Boer

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)

With the onset of the HI Parkes All-Sky Survey, a new view of the high-velocity cloud (HVC) distribution in the southern sky is being revealed. The dense spatial sampling and unbiased coverage of HIPASS gives it multiple advantages over previous surveys of the southern sky. Detailed views of the clouds' structure and large mosaics of the HI sky allow us to link individual concentrations to larger structures (such as the Magellanic Clouds and the Galaxy), providing pictorial clues as to the origins of HVCs. It is clear that HVCs cannot be uniformly assigned a single origin scenario, and must be categorised appropriately., Comment: 7 pages with 6 figures in gif format, also available with embedded postscript figures at http://msowww.anu.edu.au/~putman/pubs.html To appear in PASA, vol. 16, no. 1 (Proceedings of "HI in the Local Universe - II)

We present an analysis of high-resolution HI data of one of the cores of HVC187 Wakker & van Woerden 1991), HVC287+22+240. Structure is present down to the lowest-measurable scale (~1 arcmin) and several concentrations appear to be unresolved. Most of the cores seen at low resolution break up into smaller subcores at higher resolution. The typical volume density and pressure are estimated to be ~30/R/D(kpc) cm^{-3} and ~18000/R/D(kpc) K cm^{-3}, respectively, where R is the resolution in arcmin and D(kpc) the unknown distance in kpc., Comment: 5 pages; LaTeX; to be published in: Proceedings of the 3rd Stromlo Symposium, eds. B.K. Gibson, T.S. Axelrod and M.E. Putman (ASP Conf. Ser.)

The first results from the HI Parkes All-Sky Survey (HIPASS) provide a spectacular view of the global HI distribution in the vicinity of the Magellanic Clouds and the southern Milky Way. A 2400 square degree mosaic around the South Celestial Pole (SCP) reveals the existence of a narrow, continuous counter-stream which 'leads' the direction of motion of the Clouds, i.e. opposite in direction to the Stream. This strongly supports the gravitational model for the Stream in which the leading and trailing streams are tidally torn from the body of the Magellanic Clouds. The data also reveal additional tidal features in the Bridge region which appear to emanate from the LMC, and a distinct spiral structure within the LMC itself., Comment: 3 pages and 3 figures in gif format, LaTex, also available at http://msowww.anu.edu.au/~putman/pubs.html To appear in IAU Symp.190, New Views of the Magellanic Clouds, eds. Chu et al

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

The Fornax cluster galaxy FCC 35 shows an unusual multiply-peaked integrated HI profile (Bureau, Mould & Staveley-Smith 1996). We have now observed FCC 35 with the Australia Telescope Compact Array (ATCA) and have found a compact HI source with M_{HI} = 2.2 x 10^{8} M_{sun}, and a spatially overlapping complex of HI gas with the same mass. By combining optical observations with the HI data, we are able to identify FCC 35 as a young compact source of star formation with a nearby intergalactic HI cloud which is devoid of stars. We classify FCC 35 as a blue compact dwarf (BCD) or HII galaxy, having large amounts of neutral hydrogen, very blue colors ((U-V) = 0.1), and a low metallicity spectrum with strong narrow emission lines. Together with the presence of the HI cloud, this suggests that FCC 35 is the result of a recent interaction within the Fornax cluster., Comment: 16 pages, 7 tables, 14 figures, LaTeX, also available at http://msowww.anu.edu.au/~putman/pubs.html To appear in the June AJ