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High-Velocity Clouds: Building Blocks of the Local Group
We suggest that the high-velocity clouds (HVCs) are large clouds, withtypical diameters of 25 kpc, containing 3x10^7 M_solar of neutral gasand 3x10^8 M_solar of dark matter, falling onto the Local Group;altogether the HVCs contain 10^10 M_solar of neutral gas. Ourreexamination of the Local Group hypothesis for the HVCs connects theirproperties to the hierarchical structure formation scenario and to thegas seen in absorption toward quasars. We show that at least one HVCcomplex (besides the Magellanic Stream) must be extragalactic at adistance of more than 40 kpc from the Galactic center, with a diametergreater than 20 kpc and a mass of more than 10^8 M_solar. We discuss anumber of other clouds that are positionally associated with the LocalGroup galaxies, and we show that the entire ensemble of HVCs isinconsistent with a Galactic origin. The observed kinematics implyrather that the HVCs are falling toward the Local Group barycenter. Wesimulate the dynamical evolution of the Local Group and find thatmaterial falling onto the Local Group reproduces the location of two ofthe three most significant groupings of clouds and the kinematics of theentire cloud ensemble (excluding the Magellanic Stream). We interpretthe third grouping (the A, C, and M complexes) as the nearest HVC. It istidally unstable and is falling onto the Galactic disk. We interpret themore distant HVCs as gas contained within dark matter ``minihalos''moving along filaments toward the Local Group. Most poor galaxy groupsshould contain similar H I clouds bound to the group at large distancesfrom the individual galaxies. We suggest that the HVCs are local analogsof the Lyman limit absorbing clouds observed against distant quasars.Our picture implies that the chemical evolution of the Galactic disk isgoverned by episodic infall of metal-poor HVC gas that only slowly mixeswith the rest of the interstellar medium. We argue that there is aGalactic fountain in the Milky Way, but that the fountain does notexplain the origin of the HVCs. Our analysis of the H I data leads tothe detection of a vertical infall of low-velocity gas toward the planeand implies that the H I disk is not in hydrostatic equilibrium. Wesuggest that the fountain is manifested mainly by relatively localneutral gas with characteristic velocities of 6 km s^-1 rather than 100km s^-1. The Local Group infall hypothesis makes a number of testablepredictions. The HVCs should have subsolar metallicities. Their Hαemission should be less than that seen from the Magellanic Stream. Theclouds should not be seen in absorption against nearby stars. The cloudsshould be detectable in both emission and absorption around other galaxygroups. We show that current observations are consistent with thesepredictions and discuss future tests.
| Galaxy coordinates. II. Accurate equatorial coordinates for 17298 galaxies
Using images of the Digitized Sky Survey we measured coodinates for17298 galaxies having poorly defined coordinates. As a control, wemeasured with the same method 1522 galaxies having accurate coordinates.The comparison with our own measurements shows that the accuracy of themethod is about 6 arcsec on each axis (RA and DEC).
| Accurate Positions for MCG Galaxies
We have measured accurate celestial coordinates for 4741 extragalacticobjects, primarily drawn from a list of MCG galaxies with no recentlypublished accurate positions. The standard deviations in the newpositions depend slightly on the measurement method but are on the orderof 1.0" to 1.2". Standard deviations in the original MCG positions areconfirmed to be at the 1.5′-2.0′ level. These new positionswere integrated into NED in 1997 December.
| A catalogue of spatially resolved kinematics of galaxies: Bibliography
We present a catalogue of galaxies for which spatially resolved data ontheir internal kinematics have been published; there is no a priorirestriction regarding their morphological type. The catalogue lists thereferences to the articles where the data are published, as well as acoded description of these data: observed emission or absorption lines,velocity or velocity dispersion, radial profile or 2D field, positionangle. Tables 1, 2, and 3 are proposed in electronic form only, and areavailable from the CDS, via anonymous ftp to cdsarc.u-strasbg.fr (to130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| A Survey of Low Surface Brightness Dwarf Galaxies to Detect H i--rich Companions
We have conducted a survey for H I-rich companions in the vicinity oflow surface brightness (LSB) dwarf galaxies, objects that arecharacterized by low rates of massive star formation. This surveycomplements our earlier survey of dwarf galaxies exhibiting high starformation rates (H II galaxies). Four of the 17 LSB dwarfs in the samplehave nearby, H I-rich companions, and a total of five companions weredetected. The companion frequency is therefore ρ = 0.24, and wedetermine a lower limit on the true rate of 0.08. This is a much lowerrate of occurrence than was detected around the H II galaxies, ρ =0.57, which implies a lower limit of ρ > 0.37. Because the twosurveys were carried out under nearly identical circumstances, thisdifference is genuine and not the result of any relative bias betweenthe two samples. That H II galaxies have companions more than twice asoften as LSB dwarfs provides evidence that the bursts of massive starformation in H II galaxies are being triggered by galaxy-galaxyinteractions.
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