PDRs4All. III. JWST's NIR spectroscopic view of the Orion Bar

JWST has taken the sharpest and most sensitive infrared (IR) spectral imaging observations ever of the Orion Bar photodissociation region (PDR), which is part of the nearest massive star-forming region the Orion Nebula, and often considered to be the `prototypical' strongly illuminated PDR. We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the region to the atomic PDR -- crossing the ionisation front (IF) --, and the subsequent transition to the molecular PDR -- crossing the dissociation front (DF). Given the prevalence of PDRs in the interstellar medium and their dominant contribution to IR radiation, understanding the response of the PDR gas to far-ultraviolet (FUV) photons and the associated physical and chemical processes is fundamental to our understanding of star and planet formation and for the interpretation of any unresolved PDR as seen by JWST. We used high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science programme. We constructed a $3 spatio-spectral mosaic covering $0.97-5.27$ at a spectral resolution R of sim 2700 and an angular resolution of $0.075 To study the properties of key regions captured in this mosaic, we extracted five template spectra in apertures centred on the three dissociation fronts, the atomic PDR, and the region. This wealth of detailed spatial-spectral information was analysed in terms of variations in the physical conditions--incident UV field, density, and temperature---of the PDR gas. The NIRSpec data reveal a forest of lines including, but not limited to and recombination lines; ionic lines (e.g. and O i and N i fluorescence lines; aromatic infrared bands (AIBs, including aromatic CH, aliphatic CH, and their CD counterparts); pure rotational and ro-vibrational lines from and ro-vibrational lines HD, CO, and CH$^+$, with most of them having been detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. In addition, we observed numerous smaller-scale structures whose typical size decreases with distance from and IR lines from if solely arising from radiative recombination and cascade, reveal very high gas temperatures (a few 1000 K) consistent with the hot irradiated surface of small-scale dense clumps inside the PDR. The morphology of the Bar, in particular that of the H$_2$ lines, reveals multiple prominent filaments that exhibit different characteristics. This leaves the impression of a `terraced' transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. We attribute the different characteristics of the filaments to their varying depth into the PDR and, in some cases, not reaching the C$^+$/C/CO transition. These observations thus reveal what local conditions are required to drive the physical and chemical processes needed to explain the different characteristics of the DFs and the photochemical evolution of the AIB carriers. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star and planet formation as well as galaxy evolution.