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

TL;DR

This study investigates the kinematics of the HII region NGC 7538 using high-resolution H-alpha observations, revealing large-scale blue-shifted flows, turbulence, and complex motions influenced by stellar outflows and shocks.

Contribution

The paper provides detailed kinematic analysis of NGC 7538, highlighting the role of turbulence and outflows in its evolution, which was not previously characterized at this resolution.

Findings

Detected a blue-shifted ionised gas flow larger than 11 km/s.

Identified turbulence dominated by shocks at scales of 0.72 to 1.46 pc.

Proposed that outflows and bow shocks influence the region's kinematics.

Abstract

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.