Annu. Rev. Astron. Astrophys. 1984. 22: 445-70
Copyright © 1984 by . All rights reserved

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2.2 The Distribution of H I Gas

If we naively assume that the H I distribution expected for a normal galaxy resembles an axisymmetric, flattened disk and that its velocity field likewise shows no asymmetries, we may well conclude, after an examination of the collection of H I maps of galaxies, that there are no normal galaxies. Many, if not all, of the well-studied galaxies show deviations from circular symmetry and evidence for noncircular motions (9, 10). Detailed maps show the H I concentrated in the regions of ongoing star formation, following spiral arms ridges. The large-scale deviations from axial symmetry, warps, and even lopsidedness may be coincident in the H I, continuum, and optical light distributions. Central depressions in the H I distribution such as that seen in the Milky Way seem to be more pronounced in high-luminosity early-type systems, which possess large bulges. Peculiarities in the H I distribution occur even in galaxies that are relatively isolated in space (73).

Of particular importance to the question of environmental influence is the extent of the H I distribution, which can be an indicator of the effectiveness of external gas removal mechanisms expected to favor the outer, less tightly bound portions of the disk. Since isophotal radii are very uncertain when H I extents are measured with poor resolution, H I mapping studies of galaxies made with filled-aperture telescopes have favored the estimate of H I extent in terms of effective radii (38, 44, 63), although synthesis studies that can produce isophotal radii are accumulating (10, 93, 112a, 114). In both cases, the present data suggest that H I disks are generally modest in size, not extending past one or two Holmberg radii (20, 63). Very large H I envelopes do surround a number of systems, however (68a, 93). The better-behaved envelopes are usually found around late-type spirals and Magellanic-type irregulars, while the H I distribution in lenticulars is frequently concentrated in an annulus whose radius lies at the edge of the optical extent (80, 113). As is discussed in Section 3, H I appendages extend from the disks of galaxies believed to be undergoing tidal interactions. In a number of other cases, particularly that of NGC 628 (19, 100), the extended H I emission cannot be understood in terms of a recent encounter with a visible neighbor.

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