Abstract
Context. The interaction of expanding H II regions with their environmental clouds is one of the central questions driving the Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program FEEDBACK. Aims. We want to understand the interaction of the prototypical NGC 7538 H II region with the neighboring molecular cloud hosting several active star-forming regions. Methods. Using the SOFIA, we mapped an area of ~210′2 (~125 pc2) around NGC 7538 in the velocity-resolved ionized carbon fine-structure line [CII] at 1.9 THz (158 μm). Complementary observed atomic carbon [CI] at 492 GHz and high-J CO(8–7) data, as well as archival near- and far-infrared, cm continuum, CO(3–2), and HI data are folded into the analysis. Results. The ionized carbon [CII] data reveal rich morphological and kinematic structures. While the overall morphology follows the general ionized gas that is also visible in the radio continuum emission, the channel maps show multiple bubble-like structures with sizes on the order of ~80–100″ (~1.0–1.28 pc). While at least one of them may be an individual feedback bubble driven by the main exciting sources of the NGC 7538 H II region (the O3 and O9 stars IRS6 and IRS5), the other bubble-like morphologies may also be due to the intrinsically porous structure of the H II region. An analysis of the expansion velocities around 10 km s−1 indicates that thermal expansion is not sufficient but that wind-driving from the central O-stars is required. The region exhibits a general velocity gradient across, but we also identify several individual velocity components. The most blue-shifted [CII] component has barely any molecular or atomic counterparts. At the interface to the molecular cloud, we find a typical photon-dominated region (PDR) with a bar-shape. Ionized C+, atomic C0 and molecular carbon CO show a layered structure in this PDR. The carbon in the PDR is dominated by its ionized C+ form with atomic C0 and molecular CO masses of ~0.45 ± 0.1 M⊙ and ~1.2 ± 0.1 M⊙, respectively, compared to the ionized carbon C+ in the range of 3.6−9.7 M⊙. This bar-shaped PDR exhibits a velocity-gradient across, indicating motions along the line of sight toward the observer. Conclusions. Even if it is shown to be dominated by two nearby exciting sources (IRS6 and IRS5), the NGC 7538 H II region exhibits a diverse set of substructures that interact with each other as well as with the adjacent cloud. Compared to other recent [CII] observations of H II regions (e.g., Orion Veil, RCW120, RCW49), the bubble-shape morphologies revealed in [CII] emission that are indicative of expanding shells are recurring structures of PDRs.
Highlights
Stellar feedback in the form of radiation, winds, and supernova explosions can have positive as well as negative impacts on their environment
The ionized gas as traced by the Hα and 1.4 GHz emission is confined to the real HII region, and the [CII], 8 and 70 μm emission are partly wrapped around it
While the [CII] line can form in PDRs as well as molecular media, diffuse 8 μm emission typically stems from UV-pumped infrared fluorescence from polycyclic aromatic hydrocarbon (PAH) molecules within photon-dominated regions (PDRs)
Summary
Stellar feedback in the form of radiation, winds, and supernova explosions can have positive as well as negative impacts on their environment. The [CII] 158 μm data are provided at the IRSA/IPAC Infrared science archive https://irsa.ipac.caltech.edu/ Missions/sofia. Feedback is important for the determination of the physical processes within individual clouds and crucial in an integral sense for whole galaxies because it likely determines the general star formation efficiency (e.g., Matzner 2002; Elmegreen 2011; Hopkins et al 2014; Geen et al 2016, 2018; Kim et al 2018). With the goal to understand the impact of feedback processes onto the environment for individual resolved cases, the SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program FEEDBACK1 targets 11 galactic high-mass star-forming regions and HII regions in the ionized atomic carbon finestructure line [CII] at 158 μm (1.9 THz) and the atomic oxygen 1 http://feedback.astro.umd.edu.
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