Arecibo Mapping of Three Extended H I Galaxy Disks and Conjectures on What Lies Beyond

We present Arecibo H I mapping of three dwarf irregular galaxies with H I envelopes more than five times the diameter of their optically detectable stellar regions. These include one isolated system, DDO 154, and two systems that are apparently interacting tidally with a larger companion (DDO 137 and VCC 2062). Arecibo's superior sensitivity to extended low- level emission allows us to extend the rotation curve of DDO 154 by a factor of two beyond that obtained from synthesis mapping by other authors, and we explore the decrease in column density with radius in some detail. DDO 137 and its companion NGC 4532 exhibit an enormous horseshoe-shaped cloud with quite complex kinematics. The H I around VCC 2062 appears on the surface to be a plume swept from NGC 4694 by tidal or ram-pressure mechanisms; however, the coincidence of a faint stellar patch (revealed by our CCD imaging to be a dwarf irregular galaxy) with the peak H I column density bears striking resemblance to the "protogalaxy" H I 1225+0146. Drawing on theoretical considerations and on the observation that DDO 154 and other extended disks appear to truncate abruptly at an H I column density around 2 X 10^19^ cm^-2^, we conjecture that the total hydrogen (neutral plus ionized) column density declines more slowly than exponentially beyond that point. Extended partially ionized disks may then account for recently reported low- redshift Lyα absorption features. These considerations lead us to surmise that the total mass of gas beyond the H I edge is of the order of 1 x 10^9^ M_sun_, which is less than the mass of the standard H I disk of a typical spiral galaxy, but comparable to that of a dwarf irregular galaxy. It may be interactions within these overlapping extended reservoirs of ionized gas that lead to the observable H I features in systems such as NGC 4532/DDO 137, although the gas masses are somewhat larger for this pair.