Shaping the interstellar medium through expanding H I shells

¹ Instituto de Astronomía y Física del Espacio, Ciudad Universitaria – Pabellón 2, Intendente Güiraldes 2160 (C1428EGA), Ciudad Autónoma de Buenos Aires, Argentina
² Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
³ Instituto de Astrofísica de La Plata (CONICET-UNLP), Paseo del Bosque s/n, La Plata B1900FWA, Argentina

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 February 2025
Accepted: 24 December 2025

Abstract

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 × 10⁴⁸ and 8.4 × 10⁴⁸ 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.