The M81 Group Dwarf Irregular Galaxy DDO 165. I. High Velocity Neutral Gas in a Post-Starburst System

TL;DR

This study uses detailed radio observations to analyze the neutral hydrogen gas in the post-starburst galaxy DDO 165, revealing complex gas structures, high-velocity features, and the impact of star formation feedback on the interstellar medium.

Contribution

It provides the first detailed kinematic analysis of HI gas in DDO 165, linking gas structures and motions to recent star formation feedback effects.

Findings

Multiple HI shells and holes with sizes 400-900 pc and expansion velocities 7-11 km/s.

Distinct kinematic components suggest recent tidal interactions or blowout.

High-velocity gas features are associated with star formation feedback processes.

Abstract

We present new multi-configuration VLA HI spectral line observations of the M81 group dIrr post-starburst galaxy DDO 165. 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 ~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 ISM of DDO 165. 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 (~400-900 pc) and expansion velocities (~7-11 km/s). These structures are compared with narrow- and broad-band imaging from KPNO and HST. Using the latter data, recent works have shown that DDO 165's previous "burst" phase was extended temporally (>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. In addition to creating HI holes and shells, extended star formation 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.