THE M81 GROUP DWARF IRREGULAR GALAXY DDO 165. I. HIGH-VELOCITY NEUTRAL GAS IN A POST-STARBURST SYSTEM

We present new multi-configuration Very Large Array Hi spectral line observations of the M81 group dwarf irregular post-starburst galaxy DDO165. The Hi morphology is complex, with multiple column density peaks surrounding a large region of very low Hi surface density that is offset from the center of the stellar distribution. The bulk of the neutral gas is associated with the southern section of the galaxy; a secondary peak in the north contains similar to 15% of the total Hi mass. These components appear to be kinematically distinct, suggesting that either tidal processes or large-scale blowout have recently shaped the interstellar medium (ISM) of DDO165. Using spatially resolved position-velocity maps, we find multiple localized high-velocity gas features. Cross-correlating with radius-velocity analyses, we identify eight shell/hole structures in the ISM with a range of sizes (similar to 400-900 pc) and expansion velocities (similar to 7- 11 km s(-1)). These structures are compared with narrow- and broadband imaging from the Kitt Peak National Observatory and the Hubble Space Telescope (HST). Using the latter data, recent works have shown that DDO 165's previous burst phase was extended temporally (greater than or similar to 1 Gyr). We thus interpret the high-velocity gas features, Hi holes, and kinematically distinct components of the galaxy in the context of the immediate effects of feedback from recent star formation (SF). In addition to creating Hi holes and shells, extended SF events are capable of creating localized high-velocity motion of the surrounding interstellar material. A companion paper connects the energetics from the Hi and HST data.