Kinematics of the HII region NGC 7538 from study of the Ha line

Aims. Massive stars impact their surrounding initiating star-formation along their photo-dissociation region. Once the HII region is formed it is unclear if and how the second generation of stars impacts its aspect and evolution. Methods. We performed high spectral resolution (R ~ 23400) Ha Fabry-Perot observations in five fields covering the Galactic HII region NGC 7538 and lead profiles multi-gaussian fitting to extract the parameters as peak intensity, width and velocity. We then analyse the kinematics of the ionised gas building kinematic diagrams and second order structure functions for every field. Results. The observations reveal a general blue-shifted ionised gas flow larger than 11 km s-1 in NGC 7538, consistent with previous studies. Profiles originating from features that are dark in Ha due to extinction or from outside the region show 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 HII region itself the turbulence seems to be shock-dominated, with a characteristic scale length 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 IRS6.

💡 Research Summary

This paper presents a detailed kinematic study of the Galactic H II region NGC 7538 using high‑resolution (R ≈ 23 400) Hα Fabry‑Perot spectroscopy. Observations were carried out in October 2014 with the GHASP instrument on the 1.93 m telescope at the Observatoire de Haute‑Provence, covering five overlapping fields that together map the entire nebula (each field 5.9′ × 5.9′, pixel scale 0.68″). The data cubes were reduced with a standard pipeline: phase calibration using a neon lamp, wavelength sorting, a light spectral smoothing over three channels, and conversion to LSR velocities. To improve signal‑to‑noise, spectra were extracted from 11.7″ × 11.7″ (≈0.15 pc × 0.15 pc) apertures and fitted with a set of five Gaussian components – two fixed night‑sky lines (OH and geocoronal Hα) and up to three nebular components. The fitting employed the Minuit2 minimizer, with constraints on the night‑sky line positions (±3 km s⁻¹) and intensity ratios, while allowing the nebular Gaussians to vary freely. Only components with peak intensity > 0.2 (arbitrary units) and velocity dispersion σ between 6 and 35 km s⁻¹ were retained, yielding three reliable nebular components labeled A, B, and C.

Component A peaks near –7 km s⁻¹ and is interpreted as a local foreground ionised layer. Component B, centered at –23 km s⁻¹, corresponds to the warm interstellar medium (WIM) along the line of sight, possibly associated with the large HI shell GS112+1‑020. Component C spans –57 to –80 km s⁻¹ and traces the bulk ionised gas of NGC 7538 itself. Additional weaker features at –36 km s⁻¹ (mainly in fields 2 and 4) and –92 km s⁻¹ (confined to the nebular centre) are identified but not central to the analysis. By comparing with CO surveys (Ma et al. 2021) and HI data, the authors place these velocity components within the Perseus arm’s complex velocity structure.

To probe the dynamical state, the authors construct velocity‑position diagrams and compute second‑order structure functions S₂(l) = ⟨