The Fine Structure and Outskirts of DDO 154

Mapping of the HI disk of the isolated irregular galaxy DDO 154 with the Carray of the Very Large Array and with the 3.2' upgraded Arecibo beam ispresented. Our results show a truncation (or temporary drop) of the HI disk ata column density around 10^19 atoms cm^-2, consistent with theoreticalexpectations for the truncation produced by the extragalactic UV field. We alsodetect a marginally significant levelling off of the HI distribution along thecontinuation of the major axis at a column density near 2X10^18 atoms cm^-2.The VLA results show that the gas beyond ~6' in radius must be relativelysmooth, with no structure larger in size than ~300 pc exhibiting a densitycontrast of a factor of 10 or more. However, there is considerablefew-hundred-parsec scale structure in the gas disk at smaller radii, even welloutside the regions where there are visible stars. Two prominent cavities wellremoved from any significant stellar populations are studied. While theenergies required for evacuation are consistent with those produced by multiplesupernovae, there is no visible trace of stars within a kpc of the center ofthe larger cavity, and the smaller of the two cavities is centered just outsidethe 26.5 mag arcsec^-2 B isophote. The velocity dispersion of the gas, measuredwithin our 270 pc beam, is 7 to 8 km s^-1 throughout the disk (to 6' radius).This translates to a scaleheight of ~700 pc at the point where the rotationcurve flattens, at a radius of ~4.5 kpc. Velocity profiles are well fit bysingle gaussians at all points.Comment: 14 pages, 2 tables, 13 figures, accepted for AJ Nov. 200