Mapping of the H I disk of the isolated irregular galaxy DDO 154 with the C array of the Very Large Array (VLA) and with the 32 upgraded Arecibo beam is presented. Our results show a truncation (or temporary drop) of the H I disk at a column density around 1019 atoms cm-2, consistent with theoretical expectations for the truncation produced by the extragalactic UV field. We also detect a marginally significant levelling off of the H I distribution along the continuation of the major axis at a column density near 2 × 1018 atoms cm-2. The VLA results show that the gas beyond ~6' in radius must be relatively smooth, with no structure larger in size than ~300 pc exhibiting a density contrast of a factor of 10 or more. However, there is considerable structure on a few hundred parsec scale in the gas disk at smaller radii, even well outside the regions where there are visible stars. Two prominent cavities well removed from any significant stellar populations are studied. While the energies required for evacuation are consistent with those produced by multiple supernovae, there is no visible trace of stars within a kiloparsec of the center of the larger cavity, and the smaller of the two cavities is centered just outside the 26.5 mag arcsec-2 B isophote. The velocity dispersion of the gas, measured within our 270 pc beam, is 7–8 km s-1 throughout the disk (to 6' radius). This translates to a scale height of ~700 pc at the point where the rotation curve flattens at a radius of ~4.5 kpc. Velocity profiles are well fitted by single Gaussians at all points.
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