Mapping of the H i disk of the isolated irregular galaxy DDO 154 with the C array of the Very Large Array and with the 3 \farcm 2 upgraded Arecibo beam is presented .
Our results show a truncation ( or temporary drop ) of the H i disk at a column density around 10 ^ { 19 } 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 \times 10 ^ { 18 } atoms { cm } ^ { -2 } .
The VLA results show that the gas beyond \sim 6 \arcmin in radius must be relatively smooth , with no structure larger in size than \sim 300 pc exhibiting a density contrast of a factor of 10 or more .
However , there is considerable few-hundred-parsec scale structure 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 kpc 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 to 8 km { s } ^ { -1 } throughout the disk ( to 6 \arcmin radius ) .
This translates to a scaleheight of \sim 700 pc at the point where the rotation curve flattens , at a radius of \sim 4.5 kpc .
Velocity profiles are well fit by single gaussians at all points .