2022A&A...659A..77B

Context. The interaction of expanding HII 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 HII 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 HII 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 HII 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 HII 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 HII 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.