Shaping the interstellar medium through expanding H I shells

Aims . The goal of this study is to analyse a set of four H I shells and their potential role in triggering star formation. Methods . We analysed the H I 21-cm line and far-infrared emission distributions to characterize the four shells. To investigate star formation associated with these shells, we identified massive OB-type star candidates using Gaia data and a spectrophotometric method. Results . The H I characterisation of the four shells reveals that they are expanding structures, with expansion velocities ranging from 6 to 9 km s −1 and kinetic energies between 2.5 × 10 48 and 8.4 × 10 48 erg. Some of the shells appear to be in collision with each other. The analysis of the IR emission reveals the presence of 28 H II regions seen projected into the borders of the H I shells, some of them located at the interfaces of two shells. The spectrophotometric analysis used to identify ionising star candidates in these regions indicates a distance of 2.7 ± 0.5 kpc for 27 of the H II regions. Since their radio recombination line (RRL) velocities are consistent with the systemic velocities of the H I shells, we can infer that the shells are located at the same distance. Conclusions . The distribution of 27 H II regions along the borders of four H I shells is consistent with a scenario in which the massive stars responsible for their ionisation may have formed as a consequence of the shells’ expansion and, in some cases, their collision.