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

D. Russeil, H. Plana, P. Amram, A. Zavagno, F. Michel

TL;DR

This study addresses how massive-star feedback shapes the kinematics and evolution of the H II region NGC 7538. Using high-resolution Hα Fabry-Perot spectroscopy across five fields, the authors perform multi-Gaussian fits to extract velocity components and apply kinematic diagrams and second-order structure functions to characterize turbulence and flows. They find a general blue-shifted outflow with speeds above $11$ km s$^{-1}$, non-thermal motions and large-scale gradients, and a central kinematic configuration likely resulting from the IRS1 outflow plus a wind bow shock near IRS6, with a conical outflow of about $1.5$ pc in diameter and an ejection velocity near $-89$ km s$^{-1}$. Structure-function analysis reveals field-dependent turbulence with m2D values around 1 in several fields and a pronounced large-scale gradient, supporting a Champagne-flow–driven dynamics and highlighting the role of turbulence and outflows in shaping the region.

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.

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

TL;DR

This study addresses how massive-star feedback shapes the kinematics and evolution of the H II region NGC 7538. Using high-resolution Hα Fabry-Perot spectroscopy across five fields, the authors perform multi-Gaussian fits to extract velocity components and apply kinematic diagrams and second-order structure functions to characterize turbulence and flows. They find a general blue-shifted outflow with speeds above km s, non-thermal motions and large-scale gradients, and a central kinematic configuration likely resulting from the IRS1 outflow plus a wind bow shock near IRS6, with a conical outflow of about pc in diameter and an ejection velocity near km s. Structure-function analysis reveals field-dependent turbulence with m2D values around 1 in several fields and a pronounced large-scale gradient, supporting a Champagne-flow–driven dynamics and highlighting the role of turbulence and outflows in shaping the region.

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.
Paper Structure (9 sections, 10 figures)

This paper contains 9 sections, 10 figures.

Table of Contents

  1. Introduction
  2. NGC 7538
  3. H$\alpha$ Observations and data reduction
  4. General overview of the velocity components
  5. NGC 7538 ionised gas kinematic
  6. Kinematic diagrams analysis
  7. Structure function analysis
  8. Discussion
  9. Conclusion

Figures (10)

  • Figure 1: This image shows the position of the five observed fields overploted on the DSS-Red image of NGC 7538. Yellow isocontours are the inner and the outer (dashed) PDRs boundaries as defined by Luisi16 and delineated here from the HERSCHEL-70$\mu$m image. The blue isocontours plot the column density (from 1.6 10$^{21}$ to 1.2 10$^{24}$ cm$^{-2}$) constructed from Hi-GAL survey maps (Marsh17). In red are indicated the main infrared sources and the particular features named "Ark" and "Rim".
  • Figure 2: Example of profile decomposition. The blue and black dotted lines are the H$\alpha$ emissions (labeled A, B and C) and night-sky (labeled "H$\alpha$ geo." and "OH") components respectively. On the x-axis the velocities corrected for the free spectral range are shown in parentheses. The blue and the red plots are the observed and the fitted profiles, respectively.
  • Figure 3: Position-velocity plots of the three fitted Gaussians arranged in ascending order of intensity (from panel (A) to (C)). The black, red, green, blue and magenta correspond to the data from the fields 1 to 5, respectively.
  • Figure 4: Flux (left), velocity (middle) and velocity dispersion (right) maps for the main component (top line panels), the "$-$36 km s$^{-1}$" (middle line panels) and the "$-$92 km s$^{-1}$" (bottom line panels) components respectively. The values in the overlapping areas (particularly in the top panels) between the different fields are simply an average. The position of IRS1, IRS5, IRS6 and IRS9 are indicated as blue symbols.
  • Figure 5: Kinematic diagrams for Fields 1 to 4: (a) $\sigma$$-$ log(Flux)), (b) V$_{LSR}$$-$ log(Flux) and (c) $\sigma$$-$ V$_{LSR}$. The points highlighted in yellow, red, blue, and magenta identify groups discussed in the text.
  • ...and 5 more figures