Ulrich J. Schwarz

Distribution and Motions of Atomic Hydrogen in Lenticular Galaxies

We report the results of HI observations of eleven gas-rich SO/SOa galaxies with the Westerbork Synthesis Telescope. The majority of these galaxies have most of their hydrogen outside the optical body, in annular configurations with diameters ∼2 times the optical. These outer gas rings are often clumped, incom plete, and in approximately circular motion. They may represent the remnants of primordial, often warped, gas disks; or they may have formed from gas (or dwarf galaxies) accreted recently. Optical spectra could discriminate between these possibilities.

The Large- and Small-scale Structure of HVCs

The main question addressed in this chapter is whether or not HVCs have small-scale structure that di ers from that in low-velocity gas and the IVCs. If so, this could make it possible to distinguish HVCs from galactic gas not only by their deviating velocities, but also by their structural characteristics. Further, the physical characteristics derived from the small-scale structure may provide clues to their origin. We find that the core-halo structure that was discovered in the early seventies by Giovanelli, Hulsbosch and others, is typical for all HVCs that have been observed so far. We review some methods to characterize the small-scale structure numerically. These include the autocorrelation function, low- and high-pass filtering, and the concept of fractality. In general, these methods show a tendency for HVCs to have more pronounced small-scale structure than lower-velocity gas. However, the differences are not so pronounced that it is possible to easily distinguish between high- and low-velocity gas.

Properties of Neutral Hydrogen Clouds

In the region +19° <b< + 30°, 122° < l <143°, 21-cm line profiles were analyzed into Gaussian components, and these components combined into clouds, by means of computerized procedures. Maps of some of the clouds so defined are presented, and their properties discussed. Many clouds are elongated and/or irregular in shape. Histograms of cloud parameters are presented and subjected to statistical analysis. There is evidence for tenuous clouds of large angular extent but very low column density (N H < 1019 cm-2).

The high-velocity clouds : Galactic or intergalactic?

Nature and origin of the high-velocity clouds (HVCs) remain enigmatic after thirty years (Wakker & van Woerden 1997, ARA & A, 35, 217), owing to lack of distance information. Hypotheses range from supernova shells at 100 pc to intergalactic clouds at 1 Mpc. On statistical grounds, Blitz et al. (1996, BAAS, 28, 1349) claim that the HVCs are “remnants of Local Group formation, best explained as members of the Local Group of galaxies”. Reliable distances must come from the presence or absence of absorption at the HVCs velocity in spectra of stars at different distances. For Complex A, Mgll absorption is seen in HST spectra of the Seyfert galaxy Mrk 106, but not in the star PG0859+593 at 4 kpc (Wakker et al. 1996, ApJ, 473, 834). La Palma spectra of the RR Lyr star AD UMa at 11 kpc distance show Call absorption by Complex A at both K and H, which is lacking at 4 kpc. These absorptions are not confused with stellar metal lines. Our distance bracket 4 < d < 11 kpc places Complex A in the Galactic Halo, at 2.5 < z < 7.5 kpc above the plane; a distance similar to Local Group galaxies is excluded. The HI mass implied lies between 0.15 and 1.2 times 106 M⨀. Our result precludes local origins for this HVC. It allows an origin in a Galactic Fountain, or in interaction of infalling intergalactic material (from the Magellanic System or the Local Group) with the Galactic Halo.