Kinematics of the H II region NGC 7538 from study of the Hα line

¹ Aix-Marseille Univ., CNRS, CNES, LAM, 13388 Marseille, France
² Laboratório de Astrofísica Teórica e Observacional – Departamento de Ciências Exatas – Universidade Estadual de Santa Cruz, Ilhéus, BA 45662-900, Brazil
³ Institut Universitaire de France, 1 rue Descartes, 75005 Paris, France

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

Received: 6 June 2025
Accepted: 7 February 2026

Abstract

Aims. Massive stars impact their surroundings, triggering star formation at their edges. Once the H II region is formed, it is unclear if and how the second generation of stars impacts its aspect and evolution.

Methods. We conducted high-spectral-resolution (R ~ 23 400) Hα Fabry-Perot observations in five fields covering the Galactic H II region NGC 7538. We performed multi-Gaussian profile fitting to extract parameters such as peak intensity, width and velocity. We then analysed the kinematics of the ionised gas, building kinematic diagrams and second-order structure functions for every field.

Results. The observations reveal a general blueshifted ionised gas flow larger than 11 km s⁻¹ in NGC 7538, consistent with previous studies. Profiles originating from features that are dark in Hα due to extinction or from outside the region show a velocity dispersion larger than the one typically found for the warm interstellar medium. The analysis of kinematic diagrams and second-order structure functions reveals non-thermal motions attributed to turbulence and large-scale velocity gradients. In the direction of the H II region itself the turbulence seems to be shock-dominated, with a characteristic scale length of between ~0.72 and 1.46 pc. In this context, we propose that the kinematics of the central part of the region could be explained by the superposition of the outflow coming from IRS1 and a wind bow shock formed ahead of IRS6.